xin 



PHYLUM CHORDATA 



centre of the larva : this disappears in the adult. There is a short 

 alimentary canal with mouth and anus. The ciliated bands are lost ; 

 an outgrowth is formed to give rise to the proboscis, and a constric- 

 tion separates it from the collar ; the hinder part becomes elongated 

 and narrow to form the body of the animal ; a series of perforations 

 from the exterior give rise to the branchial pouches. A band of 

 thickened epithelium has been described on the wall of the oesophagus 

 and has been supposed to correspond to the structure termed 

 endostyle to be subsequently 

 met with in the Tunicata. The 

 collar-cord is formed by the 

 separating off of thedeeper'portion 

 of the ectoderm along the middle 

 line : or, in other species, by a 

 sinking down of the whole thick- 

 ness of the layer, which becomes 

 cut off to form a medullary plate 

 with its edges overlapped by the 

 adjacent ectoderm. 



V 



Constituting the class Ptero- 

 branchia are only the two genera 

 Cephalodiscus and Rhabdopleum. 

 These both resemble Balano- 

 glossus in having the body divided 

 into three parts or regions a 

 proboscis with a proboscis-cavity, 

 a collar with a collar-cavity 

 communicating with the exterior 

 by a pair of collar-pores (nephri- 

 dia in Rhabdopleura), and a 

 trunk with two distinct- lateral 

 cavities ; and in the presence 

 of a structure resembling a noto- 

 chord with the same relations 

 to the nervous system as in 

 Balanoglossus. They both differ 

 from Balanoglossus in having 

 the alimentary canal bent on 

 itself, so that the anal opening is 

 situated not far from the mouth ; 



tentacles arising from the collar ; and in the comparatively small 

 size of the proboscis. Cephalodiscus, moreover, has only a single 

 pair of apertures which may be regarded as representing the gill- 

 slits ; while in Rhabdopleura such openings are entirely absent, 

 their places being taken, apparently, by a pair of ciliated 

 grooves. Both forms occur in associations or colonies secreting 



Fin. 720. Cephalodiscus dodecalo- 

 phus. Gelatinous investment. (After 

 Mclntosh.) 



in the presence of arms bearing 



