NATURE 



lOI 



THURSDAY, DECEMBER lo, 1874 



ON THE CLASSIFICATION OF THE ANIMAL 

 KINGDOM* 



LINN/EUS defines the object of classification as 

 follows : — " Methodus, anima scientiop, indigitat, 

 primo intuitu, quodcunque corpus naturale, ut hoc corpus 

 dicat proprium suum nomen, et hoc nomen quascumque 

 de nominato corpora beneficio seculi innotuere, ut sic in 

 summa confusione rerum apparent!, summus conspiciatur 

 Naturae ordo." {Systema Naturce, ed. 11, p. 13.) 



With the same general conception of classificatory 

 method as Linnseus, Cuvier saw the importance of an ex- 

 haustive analysis of the adult structure of animals, and 

 his classification is an attempt to enunciate the facts of 

 structure thus determined, in a series of propositions, 

 of which the most general constitute the definitions of 

 the largest, and the most special, the definitions of the 

 smallest, groups. 



Von Baer showed that our knowledge of animal struc- 

 ture is imperfect unless we know the developmental stages 

 through which that structure has passed ; and since the 

 publication of his " Entwickelungs-Geschichte der Thiere," 

 no philosophical naturalist has neglected embryological 

 facts in forming a classification. 



Darwin, by laying a novel and solid foundation for the 

 theory of Evolution, introduced a new element into 

 Taxonomy. If a species, like an individual, is the 

 product of a process of development, its mode of evo- 

 lution must be taken into account in determining its like- 

 ness or unlikeness to other species ; and thus " phylogeny " 

 becomes not less important than embryogeny to the 

 taxonomist. But while the logical value of phylogeny 

 must be fully admitted, it is to be recollected that, in the 

 present state of science, absolutely nothing is positively 

 known respecting the phylogeny of any of the larger 

 groups of animals. Valuable and important as phylogenic 

 speculations are, as guides to, and suggestions of, inves- 

 tigation, they are pure hypotheses incapable of any 

 objective test ; and there is no little danger of intro- 

 ducing confusion into science by mixing up such hypo- 

 theses with Taxonomy, which should be a precise and 

 logical arrangement of verifiable facts. 



The present essay is an attempt to classify the known 

 facts of animal structure, including the development of 

 that structure, without reference to phylogeny, and, there- 

 fore, to form a classification of the animal kingdom, 

 which will hold good however much phylogenic specu- 

 lations may vary. 



Animals are primarily divisible into those in which the 

 body is not differentiated into histogenetic cells (Proto- 

 zoa), and those in which the body becomes differentiated 

 into such cells (Metazoa of Ha;ckel). 



I. — The Protozoa are again divisible into two groups : 

 I. the Monera (Hseckel), in which the body contains no 

 nucleus ; and 2. the Endoplastica, in which the body 

 contains one or more nuclei. Among these, the Infusoria 

 ciliaia and JlagelUita {Nociilicca, e.g.), while not forsaking 

 the general type of the single cell, attain a considerable 

 complexity of organisation, presenting a parallel to what 



per read at the Lum£an 

 .S. 



Vol. XI. — No. 267 



happens among the unicellular Fungi and Algag (e.g., 

 Mucor, Vaucheria, Caulerpa). 



II. — The IMetazoa are distinguishable, in the first place, 

 into those which develop an alimentary cavity — a process 

 which is accompanied by the differentiation of the body 

 wall into, at fewest, two layers, an epiblast and a hypo- 

 blast {Gastrccs of Ha^ckel), and those in which no ali- 

 mentary cavity is ever formed. 



Among the Gastrea;, there are some in which the 

 gastrula, or primitive sac with a double wall open at one 

 end, retains this primitive opening throughout life — as the 

 cgestive aperture ; numerous ingestive apertures being 

 developed in the lateral walls of the gastrula — whence 

 these may be termed Polystomala. This group compre- 

 hends the Spoiigida or Port/era. All other Gastreaj are 

 Monostoniata, that is to say, the gastrula develops but 

 one ingestive aperture. The case of compound organisms 

 in which new gastrulie are produced by gemmation is 

 of course not a real exception to this rule. 



In some Monostomata the primitive aperture becomes 

 the permanent mouth of the animal {A>xhceostomatd). 



This division includes two groups, the members of each 

 of which respectively arc very closely allied : — i. The 

 Ccelenterata. 2. The Scolecimorpha. Under the latter 

 head are included the Turbcllaria, the Ncmatoidea, the 

 Trematoda, the Hinidinca, the O ligoclueta, and probably 

 the Rotifera and Gcphyrea. 



In all the other Monostomata the primitive opening 

 of the gastrula, whatever its fate, does not become the 

 mouth, but the latter is produced by a secondary perfora- 

 tion of the body wall. In these Deuterostomata there is 

 a perivisceral cavity distinct from the alimentary canal, 

 but this perivisceral cavity is produced in different ways. 



1. A perivisceral cavity is formed by diverticula of 

 the alimentary canal, which become shut off from the 

 latter {Entcrocccld). 



The researches of Alexander Agassiz and of Metschni- 

 koff have shown that, not only the ambulacral vessels, 

 but the perivisceral cavity of the Echinodermata zx^ pro- 

 duced in this manner ; a fact which may be interpreted 

 as indicating an affinity with the Coelenterates (though it 

 must not be forgotten that the dendrocoele TurbcUaria 

 and many Trematoda are truly "ccelenterata "), but does 

 not in the least interfere with the fundamental resem- 

 blance of these animals to the worms. 



Kowalevvsky has shown that the perivisceral cavity of 

 the anomalous Sagitta is formed in the same way, and the 

 researches of Metschnikoff appear to indicate that some- 

 thing of the same kind takes place in Balanoglossus. 



2. A perivisceral cavity is formed by the splitting of 

 the mesoblast i^Schizocmla). 



This appears to be the case in all ordinary Molliisca, in 

 all the polychKtous Annelida, of which the Molliisca are 

 little more than oligomerous modifications, and in all 

 the Arthropoda. 



It remains to be seen whether the Brachiopoda and the 

 Polyzoa belong to this or the preceding division. 



3. A perivisceral cavity is formed neither from 

 diverticula of the alimentary canal nor by the splitting 

 of the mesoblast, but by an outgrowth or invagination of 

 the outer wall of the body {Epiavla). 



The Tunicata are in this case, the atrial cavity in them 

 being formed by invagination of the epiblast. 



