rOEM OF THE BODY OF VEETEBEATA. 413 



Todd's Cyclopoadia. I. — Milne-Edwakds, Alph., Rech. siir les ossemens fossiles des oiseaux 

 Paris, 18CC. — Alis, E., Appareil looomoteui- des oiseaux. Paris, 1874. 



Mammalia : Meckel, J. Fk., Oniithorhynchi paradoxi descriptio anatomica. Lips. 1826. — Vbolik 

 (Dendrohigus), Verliandel. d. Kon. Acad. Amsterd. V. — Gurlt, Handb. d. vergl. Anat. der 

 Haussiiugethiere. •! Aufl. Berlin, 18G0. — Fe.vnck, L., Anatomie der Hausthiere. Stuttgart, 1871. 

 — BuAKDT (Lama), Mi^m. Acad. St. Pc^tersbourfr, 1811. — Owen (Giraiie), Transact. Zool. Soo. II. — 

 The same (Rhinoceros), Transact. Zool. Soc. IV. ii.— MiLNE-EDW.iBDs, Alph. (Moschired), Ann. 

 sc. nat. V, II. — MuRiE, J. (Manatus). Tr. Zool. Soo. VIII. — Oampek, Observations sur la structure 

 intime et le squelette de Ci^tac6es. Paris, 1820. — Rapp, Die Cetaceeu. Stuttgart u. Tiibingen, 

 1837. — Vrolik, W., Natuur- en ontleedkund. Bcschouwing van d. Hyperoodon. Haarlem, 18-18. 

 EsCHRiCHT, Untersuch. iiber die norcUsohen Walthiere. Leipzig, 1S19. — Mhrie, J. (Globio- 

 cephalus, Otaria, Tricliechus) , Tr. Zool. Soo. VII. VIII. — Rapp, Anatom. Untersuchungen iiber 

 die Edeutaten. 2 Aufi. Tiibingen, 1852. — Owen (Myrmecophaga jubata), Tr. Zool. Soc. IV. — 

 HrsTL (Chlamydopliorus truncatus), Denkschr. d. Wien. Acad. IX. 1855. — Pouchet, G., M^m. 

 sur le grand Fourniilier. Paris, 1874. — Pallas, Nov. spec, quadrup. e gliriumordine. Erlangen, 

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 the subject, 



Form of the Body. 



§ 316. 



The external metamerism disappears, but dorsal and ventral 

 surfaces can generally be distinguished; the entrance into the 

 nutrient canal is placed near the anterior pole of the long axis of the 

 body, and on the ventral surface ; the anus is also ventral, but is at 

 some distance from the aboral pole. Three great regions can be 

 made out even in the body of the lowest divisions. The anterior one 

 contains a respiratory chamber formed from the nutrient canal, and 

 is consequently distinguished by clefts in the sides of the body-wall. 

 It carries the higher sensory organs, and in the Craniota gives rise, 

 by concrescence and differentiation, to a head. 



The second portion, which in Amphioxus is not sharply marked 

 off on the dorsal surface from the preceding one, forms the trunk, 

 which encloses the coelom and its contents ; this is only marked off 

 from the last or caudal portion of the body by the anus ; so that 

 from the outside there is not much difference between them. 



We have already met with these divisions in the Tunicata. In the 

 Ascidian larvae (cf. Fig. 208), the most anterior one, which later on 

 forms the principal portion of the body, contains the rudiments of 

 the respiratory cavity, and the portion of the nervous system which 

 contains the sensory organs. Connected with this is a faintly 

 separated tract with the enteric tube, and this passes almost directly 

 into the caudal portion. The earliest characters of the embryonic 

 head, or of its equivalent in all Vertebrata, point to its being, 

 phylogenetically, the most ancient portion of the body, and serve as 

 a finger-post to the path of development of the Vertebrate body. 



With the development of the head and of the organs differen- 

 tiated in and on it, the body of the Vertebrata acquires characters, 

 which, externally, separate it well off from the Invertebrata ; the 

 value of these is clear when we take note of the large number 



