Tải bản đầy đủ - 0 (trang)
Chapter 2. A taxonomist’s view on classification

Chapter 2. A taxonomist’s view on classification

Tải bản đầy đủ - 0trang

18 T. HANAI



relevant or irrelevant only in reference to a given hypothesis, but not in reference to a given problem.” He further noted that “without such hypothesis, analysis and classification [for analysis]

are blind.” If one wants to clarify the relationship of a given taxon based on the method of hypothesis testing, the first thing one must do is to pinpoint the problem, i.e., the relationships among

a given group of organisms, and then give a tentative answer to that problem, i.e., the hypothesis,

and, further, subject it to empirical tests. In this case, classification is a poor, or at best, optimized

presentation of the hypothetical proposition in a hierarchical manner, no matter what kind of

relationships one may choose to study.

On the other hand, the observations and recordings of all the facts, and their analysis and

classification, done so laboriously by inductivists, no longer had a purpose when the effort to discover the plan of creation ended in failure. However, the classification system that had become

highly developed by the nineteenth century remained as a general frame of reference. As explained

by Mayr (1972: 94), the system provides a sound foundation for all comparative studies in biology, and serve[s] as an efficient information storage and retrieval system. Thus it is somewhat

similar to an information data base in a computer system. As has been noted by Mayr (1974: 96)

and quoted elsewhere by many authors, classification is thus considered to be best, when it “allow[s]

the greatest number of conclusions and predictions’’ following the concept of the best possible

classification by Mill (1874: 466, 467). Thus, the status of classification has turned out to be a

product of scientific systematization.

Before going into the discussion on scientific systematization, the relationship between classification and classificatory theories will be considered. Hempel (1965: 139) noted that the construction of a classification “may be considered as a special kind of scientific concept formation’’

in which classificatory theories have played an important role. Indeed, the relationship between

the formation of classification and the formation of underlying theory is quite similar to that of concept formation and theory formation. The classificatory procedure requires theories to provide

criteria for distinguishing similarities and differences among the organisms under consideration,

and the resulting classificatory schemes differ according to their classificatory theories. Therefore,

the classes thus formed are considered by Hempel to be an extension of the corresponding classificatory concept.

Early biological classifications were based directly on observed morphological characteristics,

and each class was grouped through a procedure of comparative anatomy. After Darwin, however, classification came to rest on a far more theoretical basis, that is, on a phylogenetic and genetic

basis. We have observed, for example, a change from the concept held by Linnaeus, which was

strongly influenced by Aristotle’s work, to the species concepts advanced by Dobzhansky (1937 :

321), Mayr (1942: 120), Simpson (1961 : 153), and others, being defined in phylogenetic and/or

genetic terms.

The change to the new species concept is effective not only at the species level of classification,

but also influences higher categories of classification, because the secondary effects of speciation

persist even after populations reach species level. In either case, morphological and other characteristics become simply the observational criteria for the assignment of individuals to a classificatory

category systematized according to phylogenetic, genetic and other theories. Thus it seems to me

that biological classification is not constructed to test phylogenetic and genetic theories, but is, on

the contrary, constructed based on these theories.

In general, the differences between the theories underlying classification are reflected in the

differences in the nomenclature of the classes. In the case of biological classification, however,

binominal nomenclature, for example, has long been in use since Linnaeus in spite of the radical

change in theories behind classification, This is perhaps because we have treated “objective existence in nature as ‘carving nature at the joints,’ ’’ to use Hempel’s phrase (1965: 147). However,



A Taxonomist’s View on Classification 19



a profound change seems to have occurred in the meanings implied by binomials and this is reflected in the diagnosis and description of organisms.

The characteristics chosen for diagnosis and description are those considered to be important

for the particular theory behind a classification. The differences in classificatory theories issuing

from the corresponding concepts give rise to differences in scientificvocabulary selected for description. As biological understanding of organisms advances, the shift from a predominantly descriptive to an increasingly theoretical emphasis is reflected in the replacement of the purely descriptive terms by terms of theoretical background, again as explained by Hempel (1965: 140).

To cite some examples from previous lectures presented at this ostracod symposium, Harding

(1964: 1, 2), at the Neapolitan symposium emphasized the continuity of the ostracod carapace

and pointed out that



the shell is not made up of two separately secreted entities, as is the shell of a bivalve mollusk . . . [but] the two calcified valves of an ostracod shell and the soft cuticle jointing them

together are one continuous piece of cuticle . . . The cuticle forms one continuoussheet, soft

in some places and hard in others according to the functional needs of the part concerned.

Thus the attached margin of the carapace is not considered to be the margin of the carapace,

but to be the crease or joint along the median line of the carapace. Further, the ligament is elastic,

but it does not take part in the opening force of the carapace.

At the Hull symposium, Kornicker (1969: 109) illustrated details of the calcified portion of

the cuticle (Text-fig. 1). He traced the inner margin of the “duplicature” into the dorsal ligament.

Thus, the structures of the free margin which is the exterior of the inner margin of the “duplicature”

lost their homologous counterpart on the exterior surface of the dorsal ligament. This is perhaps

because structures of the free margin of one valve converge on the anterior and posterior juncture

of the attached margin and then probably continue to those of the opposite valve. Thus Kornicker’s

work gave strong support to Harding’s idea that the ostracod shell is a calcified portion of one

continuous piece of cuticle, and the ligament is a soft zone demanded by functional requirement.



*

%

; ’



*

2



...

...



..... .........

.....

........



fi



T



3



.......

.....

........

.......

.....

.........

.........

.............

..................

..............

............

.....................

.............



4



TEXT-FIG.

1-Figures showing successive sections of the marginal area of the free margin (1-3) and the dorsal

hinge area (4). When one traces the inner margin from the free margin towards the hinge area (broken line), the

inner margin moves from the inside of the carapace to the outside of the carapace. Dotted area shows the part

occupied by the animal body. Thus the hinge is divisible into exterior and interior portions with the ligament

between. After Kornicker, 1969, modified.



20 T. HANAI



A



C



E

TEXT-FIG.2-Three types of hingement in terms of the upper and lower elements i.e., exterior and interior

portions. A,B. Leprocythere pellucidu (Baird, 1850), left valve has a median groove termed the ‘containant

(Hanai, 1957, p. 432)’ to receive the ridge of the right valve. The containant may well be an exterior element. C,

D. Pectocyfhere quudrungulufuHanai, 1957, anterior and posterior tooth of the left valve is divided into upper

and lower, i.e., exterior and interior elements. Note that the median element of the left valve is a ridge and

may well be an exterior element. E, F. Hemicythere sp., typical amphidont hingement for comparison.



The concept of distinguishing interior and exterior surfaces of the shell, derived from this

one continuous sheet theory of the ostracod carapace, introduced a useful operational criterium to

understand the arrangement and disposition of minor structures on the marginal area of the shell.

The so-called “calcified portion of the inner lamella” or “duplicature” is not related to the inner

lamella, but turns out to be actually an infold portion of the shell margin, and the zone of concrescence is considered to be the axial plane of the marginal folding along the free margin of the

shell. Kornicker (1969:119) explained that the hinge structure may be exterior, interior or both

and the concept of distinguishing interior and exterior hinge structures might prove useful for

classification. The upper and lower subdivision of the hinge structure that was described without

any knowledge of its biological significance by myself (1957: 473) thus has a solid theoretical basis

may be called the exterior and interior elements, respectively (Text-fig. 2). Indeed, it is possible to

offer a description of characters even if little or nothing is initially known about their biological

function.

The single continuous sheet theory of the ostracod carapace covers wide explanatory and predictive areas. Explanation covers not only the structures from which the theory is derived, but also

indirect phenomena of a different nature, Examples may include the dominantly intact whole

carapace occurrences of the oldest group of ostracods, Bradorina. In general, the arrangement

of normal pores over the ostracod carapace surface is concentric with one center.

To give an example, four types of normal pores are distinguishable on the carapace of Cythere

omofenipponica (Text-fig. 3). The first type (PI. 1, fig. 1) is distributed in the marginal area along

the free margin. Seta is stout in its lower half, tapers rapidly and is easily twisted in its upper half,

and terminates somewhat like a tube. The pore of this type of setae has irregular decoration around

its opening. The second type (Pl. 1, fig. 2) is also distributed along the free margin, but is more or



PLATEl-Various kinds of trichoid sensillia which open on-the outer surface of the ostracod carapace.

Figs. 1-5. Cythere omotenipponica Kanai, 1959.

Fig. 6. Keiiellu bismensis (Okubo, 1972).

ISingle and double scale bars indicate 5 micrum and 0.5 microns respectively.J



22



TEXT-FIG.3-Cythere omotenipponicu Hanai, 1959, lateral view. [scale bar indicates 100 microns.]



TEXT-FIG.

&Distribution of trichoid sensillia along the ventral margin of Cythere omotenipponicu Hanai, 1959.

An example of type 1 is shown in Plate 1, fig. 1 and type 2 in Plate 1, fig. 2.



A Taxonomist’s View on Classification 23



less inside the area of the zone of the first type (Text-fig. 4). Setae of this type bifurcate near their

base, and have branches of similar size that extend widely apart parallel to the free margin. The

branched setae taper gradually to a sharp point. Pores of this type are small and simple with no

decoration. The third and fourth setal types are distributed widely in the central and dorsal area,

except along the hinge margin, and correspond to the two types of setae or two types of pore canals

that coexist on one carapace and have been described elsewhere. Type three (Pl. 1, fig. 3) is a long

stout seta that tapers gradually and terminates with a pointed end. The associated pore has a wide

and clearly rimmed lip. Pore canals of the fourth type correspond to sieve type pore canals. The

fourth type of seta seems to include two forms. One is a stout seta without branches, and the other

(Pl. 1, fig. 4) is a stout seta with one slender branch near its base. The setal pore of the former seems

to occupy on margin of the sieve plate, whereas that of the latter emerges from the central area of

the sieve. The nature of the stout seta of both forms seems similar to the first type of seta. Further,

a homologous relationship of the structural elements of so-called normal pore canals and radial

pore canals was predictable considering the concept of the marginal infold. This relationship was

proved by Okada (1982: 254). Thus, the diversity of pore canal structures that has been encountered in relation to the position on the carapace are concordant with the continuous nature of

the ostracod carapace across the hinge margin.

Description of setae and pore canals has for the most part been purely morphological. I recall

a period just after the invention of the scanning electron microscope when many new terms were

proposed to describe the minute details of the ostracod carapace without understanding anything

of their biological significance. However, since these pores are sensillium, theorization to systematize these morphological characteristics as receptors may be desirable for utilization in a theory

that impacts on the establishment of a biological classification. Since ostracods are marine crustacea, the most essential information may concern their physical environment e.g. nature of

substratum, water movement, chemical nature of the water, and water temperature.

The presence of mechanoreceptive sensillia with two types of sensory setae (Pl. 1, fig. 6) has

been known since the 1894 monograph by G.W. Muller. One is thick and long, and is interpreted

as a receptor for direct touch by a solid object. The other is fine and short, and is sensitive to more

delicate elements such as the movement of the surrounding water. This has been well established

by Hartmann (1966: 113-117), Keyser (1983: 649-658), and Okada (1983: 640-648).

The presence of chemoreceptive sensillia has been doubted. However, Sandberg (1970: 120)

illustrated the sensory seta of a sieve pore of “Aurilu” conrudifloridunu. The seta is dendritic with

a stout upright stem that terminates like a tube. Cythere omotenipponica also have setae (Pl. 1,

fig. 5 ) that terminate somewhat like the trunk of an elephant, if, of course, we disregard size. Finally,

in a current study Kamiya has found in Loxoconchujuponica an arm-like structure on the tip of the

seta (Pl. 2, fig. 1, base; P1. 2, fig. 2, tip). The general behavior of ostracods in response to the

chemical nature of sea water, the dendrites with ciliary structure inside of the seta, together with

the tube-like nature of the seta, make the presence of chemoreceptive sensillia on the carapace

highly likely.

At the Saalfelden symposium, Rosenfeld and Vesper (1976) showed the relationship between

the form of the sieve pore and the salt concentration in water. At the Houston symposium, the

secretion or excretion of a certain hydrophobic substance was suggested as the function of sieve

pores by Keyser (1983: 654). Yet, Miiller’s old hypothesis that the sieve pores may be a light

sensory organ, as is suggested by the distribution of underlying pigment cells in Loxoconcha stellifera, still needs to be reckoned with, but at this time in relation to the possibility of their being a

thermoreceptor. In Loxoconcha juponica, there are structures shaped like a bundle of test tubes,

which open distally to form a sieve plate thrusting through the shell layer and inserting their blind

proximal ends fairly deeply into the tormogen cell (PI. 2, fig. 4). The proximal tips of the test-tube-



A Taxonomist’s View on Classification 25



like structures converge upon an area surrounding the core of the dendrites (Pl. 2, fig. 3). In 1982,

my colleagues and I described a peculiar shaped pore (Pl. 2, figs. 5 and 6). We called this type

of pore “Ben-type pore,” because the general shape of the cap remined us of the device used to

prevent the back flow of dirt in the hole of a Japanese-style water closet (Benjo in Japanese) in

the Shinkansen bullet trains. It has been suggested that the purely morphological term “Ben-type

pore” replaces the theoretical term “exocrine pore.” However, such change of terminology requires great circumspection. Are there any other species that have exocrine pores or something

similar to them? Why does the exocrine pore, which is supposed to have high inside pressure,

need a cap with this peculiar structure which at a glance looks useful for protection against the

back flow of dirt? These questions require explanation. The presence of a thermoreceptive cell

associated with other receptors may be even still more undeniable in relation to the sieve pores.

Water temperature is quite likely to be a vital environmental factor, especially for ostracods living

in shallow water environments such as tide pools. Yet, in general terms, theorization of sensillia

on the ostracod carapace and, hence, the simplification of vocabulary seems only to be a matter

of time.

Hempel (1966: 94) notes that “scientific systematization requires the establishment of diverse

connections, by laws or theoretical principles, between different aspects of the empirical world,

which are characterized by scientific concepts.” The examples given above are extremely minor

in terms of the classification of the ostracods, yet they will have direct or often indirect but definite

effectson the classification, because what we try to classify is not the very nature of ostracods, but

the ostracods that one comprehends, and the theorization exemplified above has no doubt advanced the comprehension of ostracods. If one applies Hempel’s view on the interrelationships between the scientific concepts and theoretical principles to the classification, then the more theoretical principles converge directly and indirectly upon the classification thus formed, and the stronger

will be its systematizing role. Of course, simplicity in the sense of economy will be an important

.feature of the classification, when one deals with the same amount of information. Argument between classifications of different information content seems to me of little substance. It seems to

me that many major and minor theoretical principles converge directly and indirectly upon the

classification of the ostracods and operational criteria for classification have been modified. In

this way the classification of ostracods has been advanced with growing knowledge on ostracods

ever since Monoclus of Linnaeous.



REFERENCES

DARWIN, FRANCIS ed.



1888. The life and letters of Charles Darwin including an autobiographical chapter. 3v. John Mur-



ray, London.

DOBZHANSKY, TH. 1937. Genetics and the origin of species. 364pp. Columbia Univ. Press, New York.

~ U L D1.0.

,

1980. The promise of paleobiology as a nomothetic, evolutionary discipline. Paleobiology, 6 (I), 96-118.

HANAI, T. 1957. Studies on the Ostracoda from Japan. I. Subfamily Leptocytherinae. n. subfam. Jour. Fac. Sci. univ.



Tokyo. sec. 2. 10 (3), 431-468, pls. 7-10.



-1957. Studies on the Ostracoda from Japan. 11. Subfamily Pectocytherinae n. subfam. Ibid., 10 (3), 469-482, pl.

11.

-1982. Introductory note on the studies on Japanese Ostracoda. In HANAI, T. (ed.). Studieson JapaneseOstracoda.

Univ. Mus. Univ. of Tokyo, Bull., 20, 1-13.



2-Outside(Figs. 1, 2) and inside (Fig. 3) views of minute structures of the trichoid sensillia including

tormogen cell (Fig. 4), and other types of pore openings on the ostracod carapace (Figs. 5,6).

Figs. 1-4. Loxoconcha japonica Ishizaki, 1968.

Figs. 5,6. KeijelIa bisanensis (Okubo, 1975).

[Single and double scale bars indicate 5 microns and 0.5 microns respectively.1



PLATE



G. 1966. Ostracoda. In Bronns Klassen und Ordnungen des Tierreichs, 5. Bd. (Arthropoda), 1. Abt. (Crustacea), 2. Buch, ZV. Teil, 1-4. Lieferung, 1-786. Akademische Verlagsgesellschaft, Leipzig [in German].

HARDING, J.P. 1964. Crusracean cuticle with reference to the ostracod carapace. In PIJRI, H.S. (ed.). Ostracods as ecological and palaeoecological indicators. Pubbl. star. 2001. Napoli. 33 suppl., 1-23.

HEMPEL, C.G. 1965. Aspect of scientific explanation and other essays in thephilosophy of science, 504pp. The Free hess,

Macmillan Publishing Co., New York.

-1966. Philosophy of natural science. Foundations of philosophy series, 116pp. Prentice-Hall, Englewood Cliffs,

N.J.

KEYSER, D. 1983. Ultrastructure of carapace-sensilla in Aurila convexa (Baird, 1850) (Ostracoda, h t a c e a ) . In MADDOCKS, R.F. (ed.). Applications of Ostracoda, 649-658, Department of Geoscience, Univ. of Houston, Houston,

Texas.

KORNICKER, L.S. 1969. Relationship between the free and attached margins of the myodocopid ostracod shell. In

MALE, J.w. (ed.). The taxonomy. morphology and ecology of recent Ostracoda, 109-135, pls. 1-7. Oliver and

Boyd, Edinburgh.

MAYR, E. 1942. Systematics and the origin of species. Columbia Univ. Press, New York. (Reprinted 1964. Dover Publishers, 334 p ~ . )

-1982. Thegrowth of biological thought. Diversity, evolutionand inheritance,974pp. Belknap, Harvard Univ. Press,

Cambridge, Massachusetts.

MILL, J.S. 1874. A system of logic, ratiocinative and inductive, being a connected view of theprinciples of evidence and

the methods of scientific investigation. 8th ed. 2v. Longsman, Green and Co., London.

M~~LLER,

G.W. 1894. Die Ostrakoden des Golfes von Neapel und der angrenzendenMeersabschnitts.Naples Stn. Zool.

Fauna Flora. Golfes Neapel. Monographie 31, 404 pp. [in German]

NORTI~ROP,F.S.C. 1947. The logic of the sciences and the humanities. 402pp. Macmillan, New York.

OKADA, Y. 1982. Ultrastructure and pattern of the carapace of Bicornucythere bisanensis (Ostracoda, Crustacea). In

HANAI, T. (ed.). Studies on Japanese Ostracoda. Univ. Mus., Univ. of Tokyo, Bull. 20, 229-255, pls. 16-30.

1983. Ultrastructure and function of pores of ostracodes. In MADDOCKS, R.F. (ed.). Application of Ostracoda,640648, 3 pls. Department of Geoscience, University of Houston, Houston, Texas.

ROSENFEDL, A. and VESPER, B. 1977. The variability of the sieve pores in recent and fossil species of Cyprideis torosa

(Jones, 1850). as an indicator for salinity and paleosalinity. In L~FFLER,H. and DANIELQPOL, D. (eds.). Aspect

and ecology and zoogeography of recent and fossil Ostracoda, 55-67, 1 pl. Junk, The Hague.

SANDBERG, P.A. 1970. Scanningelectronmicroscopy of freeze-dried Ostracoda(Crustacea). Trans. Amer. Microsc. SOC.,

89 (l), 113-124.

SIMPSON, G.G. 1961. Principles of animal taxonomy, 247pp. Columbia Univ. Press, New York.

SYLVESTER-BRADLEY, P.C. 1973. The new palaeontography. In SYLVESTER-BRADLEY, P.C. and SIVETER, D.J. (eds.). A stereoatlas of ostracod shells. 1 (l), 1-4.

1973. Universal Decimal Classification and retrieval of taxonomicdata. In SYLVESTER-BRADLEY, P.C. and SWETER,

D.J. (eds.). Ibid. 1 (l), 5-22.



HARTMA",



-



-



I1



Taxonomy



This Page Intentionally Left Blank



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 2. A taxonomist’s view on classification

Tải bản đầy đủ ngay(0 tr)

×