Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/64

54 highly probable that the apparent splitting of the meso- blast in the latter, after all, represents the mode of develop ment of the pleuroperitoneal cavity which obtains in the former, and, thus, that the Vertebrata are not schizoccelous, but epiccelous. Whether this suggestion will turn out to be well based or not, must be decided by the embryologi- cal investigations specially directed to this point : but that there should be any essential difference between Amphioxus and other Vertebrata, in the manner in which the pleuro peritoneal cavity is formed, is highly improbable. The distance between Amphioxus and other vertebrate animals, which has hitherto been generally supposed to exist, has been greatly diminished by recent investigations. So far from being devoid of a brain and of a skull, the regions of the cerebro-spinal axis and of the neural canal, which answer to those organs in the higher Vertebrata, are, in proportion, extremely long in Amphioxus, as they are in all vertebrate embryos. But, in Amphioxus, the head retains throughout life a segmentation comparable to that of the rest of the body, while, in the higher Vertebrata, almost all traces of these distinct segments are very early lost. Moreover, in Amphioxus, the renal apparatus, so far from being absent, is represented by a comparatively large structure, and nothing is wanted to equip it with all the organs found in a young Marsipobranch, but auditory sacs, which, however, it must be remembered, make their appearance late in the Lamprey. With all this, the gap be tween Amphioxus and the Marsipobranchii is undoubtedly more considerable than that between the Marsipobranchii and other fishes, and it may represent a primary division of the class Pisces, which, from the segmentation of the skull, may be termed the Entomocrania, as opposed to the rest, in which the primary segmentation of the skull is almost completely effaced, and which may therefore be designated Holocrania. It has been stated above that the great majority of the Metazoa pass through the Gastrula condition, and belong to the division of the Gastrcece. In some members of this division, however, the alimentary canal may be rudimentary, as in sundry male Eotifera and in the Gordiacei among the Nematoidea, and yet these are so closely allied to other forms possessing fully developed digestive canals, that it is reasonable to regard their rudimentary alimentary appa ratus as absorbed. In two groups, however, the Cestoidea and the Acanthocephala, there is no trace of an alimentary canal either in the embryo or in the adult. From the point of view of phylogeny, this fact may be interpreted in two ways. Either the alimentary canal which once existed has aborted, and the Cestoidea and Acanthocephala are modified Scolecimorpha, or these para sites have not descended from Gastrcece, but have passed into their present condition directly from a J/orw/a-like form of Metazoon. In the latter case they will form a division of Agastnece, apart from the other Metazoa.

 The subjoined synopsis indicates the general relations of the different groups of the Animal kingdom, in accordance with the views which have been put forward in the pre ceding pages. Those who are familiar with the existing condition of our knowledge of animal morphology, will be aware that any such scheme must needs, at present, be tentative and subject to extensive revision, in correspondence with the advance of knowledge. Nor will they regard it as any objection to the scheme of classification proposed, that the divisions sketched out may be incapable of sharp definition the constant tendency of modern investigations being to break through all boundaries of groups, and to fill up the gaps between them by the discovery of transitional forms. In the place of assemblages of distinctly definable groups, which it has hitherto been the object of the Taxonomist to define and co-ordinate in precise logical categories, we are gradually learning to substitute series, in which all the modifications by which a fundamental form passes from lower to higher degrees of organic complication, are summed up.

 

  ANIMALIA.

I. PROTOZOA.

i. MONERA.

ProtamcebidiE. Protomonadidae. Myxastridre, &c.

ii. ENDOPLASTICA.

Amcebidse. Flagellata. Gregarinidse. Acinetida. Ciliata. Radiokria.

A. Gastraeae.

i. POLYSTOMATA.

Porifera (or Spongida).

ii. MONOSTOMATA.

1. ARCH.EOSTOMATA. a. Scolecimorpha.

Eotifera.

Turbellaria.

Trematoda.

Nematoidea.

2. DEUTEROSTOMATA. a. Schizocoela.

II. METAZOA.

Hirudinea. Oligochseta.

8. Coslenterata.

Hydrozoa. Actinozoa.

b. Enterocoela.

Annelida (Polychceta). Gephyrea.

Arthropoda. Mollusca.

B. Agastrsese (provisional).

Acanthocephala. Cestoidea.

Brachiopoda (?) Polyzoa (?)

Enteropneusta. Chcetognatha. EchinoderiKata.

c. Epicoela. Tunicata. Vertebrata.