IV. VERTEBRATA 



449 



a blind tube, surrounded by tough membrane and thickened beneath, which 

 extends from the pharynx into the proboscis. Embryology throws little light on 

 the problem. Some species have a direct development (fig. 507, B, C), while 

 others have a larva (Tornaria, A) which so resembles the larvae of certain echino- 

 derms that it was long held to belong to that phylum. The chief resemblances 

 are in the relations of the ciliated bands to the alimentary tract and in the pres- 

 ence of the proboscis cavity, which, like the ambulacral system, opens to the 



exterior. The old genus Balano- 

 glossus* has recently been sub- 

 divided. Two deep-sea forms, 

 Cephalodiscus and Rhabdoplciira, 

 have the same type of 'notochord,' 

 and the first has a pair of gill slits; 

 in other respects these are strikingly 

 Polyzoan in appearance. 



SUB PHYLUM IV. VERTEBRATA. 



In the vertebrates only the 

 internal segmentation occurs. 

 This is shown most clearly in 

 the lower Vertebrata, in the 

 muscles (myotomes, myomeres), 

 the myosepta which separate 

 them, and the protovertebra? from 



FIG. 506. 



FIG. 507. 



FIG. 506. Balanoglossus kowalewskii* (from Korschelt-Heider, after A. Agassiz). 

 db, dorsal blood-vessel; e, proboscis; g, sexual region; k, gill region; kr, collar; vb, 

 ventral blood-vessel. 



FIG. 507. A, Tornaria larva of Balanoglossus (after Morgan), a, apical plate; 

 ac, preoral part of ciliated band; be 1 , be'-, be*, ccelomic pouches; m, mouth; p, postoral 

 part of ciliated band B, C, two stages of Balanoglossus with direct development 

 (after Bateson). a, anus,, be, branchial clefts; c, collar; dc, digestive part of alimentary 

 canal; in, intestine; nc, 'notochord'; p, proboscis. 



which they arise; in the nerves (neurotomes), the skeleton (sclerotomes), 

 the blood-vessels, and in the excretory organs (nephrotomes). In the 

 higher vertebrates this metamerism is best seen in the embryonic stages. 

 In part the absence of external segmentation has its cause in the heter- 

 onomy (p. 126) of the body and the obliteration of segmental boundaries, 

 consequent upon the union of somites into regions, of which at least three 



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