CLASSiriOATION OF THE ANIMAb KINGDOM. 211 



' 2. The Deuterostomata. — In the remaining Grastrese the em- 

 bryo develops a secondary mouth as a perforation of the body- 

 wall, the primary aperture sometimes becoming the anus and 

 sometimes disappearing. 



The Schizocoela. — -Of these Metazoa Deuterostomata there are 

 some which follow the mode of development of the Oligochista 

 and Hirudinea very closely, so far as the formation and segmen- 

 tation of the mesoblast is concerned ; though the question whether 

 this segmented mesoblast arises from the epiblast or the hypo- 

 blast, has not been exhaustively worked out. These are the An- 

 nelida Polyehseta. 



It is a very general, if not universal, rule among these animals, 

 that the Gastrula is formed by invagination, and that the aper- 

 ture of invagination persists as the anus of the adult. Almost 

 universally, again, the outer surface of the Gastrula is provided 

 with cilia, by the working of which it is actively propelled through 

 the water in which it lives ; and these cilia usually become re- 

 stricted to certain areas of the body, in the form of zones trans- 

 verse to its long diameter. In this respect the larvae of some 

 Grephyrea present similar features. Moreover setge, developed 

 in involutions of the ectoderm, are very generally present, espe- 

 cially on the limbs, when such exist. Some are apodal ; some 

 possess symmetrically disposed setae in each segment of the body ; 

 and in many, true though rudimentary limbs {parapodia), one 

 pair for each segment of the body, occur. In a few of the highest 

 forms (e. g. Folynoe) some of the anterior limbs are turned for- 

 wards, and lie at the sides of the mouth, foreshadowing the jaws 

 of the Arthropoda. In some, a process of the ectoderm, in the 

 region of the head, gives rise to a cephalic hood or mantle. A 

 perivisceral cavity occupies the space between the wall of the 

 body and that of the alimentary canal, and, so far as is known, is 

 invariably formed in the substance of the mesoblast, by a sort of 

 splitting or divarication of its constituent cells, whence it would 

 seem to be a rehabilitation of the primitive blastocoele. The great 

 majority of the Folychceta possess the so-called "segmental 

 organs" — variously formed tubes, which open on the surface of 

 the body, on the one hand, and, usually, into the perivisceral 

 cavity on the other. Not unfrequently these, or some of them, 

 play the part of conduits of the generative products. 



The lower Arthropoda closely resemble the Polychseta in their 

 development, except that the food-yelk is usually large, the ali- 



lilNlf. JOirEN, — ZOOLOGY, YOL. XII. 15 



