XIII 



PHYLUM CHORDATA 



it from the collar; the hinder part becomes elongated and 

 narrow to form the body of the worm ; a series of perforations 

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

 of thickened epithelium has been described as developed 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 (p. 14). The collar-fold is formed by the separa- 

 ting off of the deeper portion 

 of the ectoderm along the 

 middle line : or, in other species, 

 by a sinking down of the whole 

 thickness of the layer, which 

 becomes cut off to form a 

 medullary plate with its edges 

 overlapped by the ectoderm. 



Usually associated with 

 Balanoglossus are two aberrant 

 animals - - Cephalodiscus and 

 RhaMopUum - - formerly re- 

 garded as Polyozoa. These 

 both resemble Balanoglossus 

 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, and a trunk with two 

 distinct lateral cavities ; and in 

 the presence of a structure re- 

 sembling a notochord with the 

 same relations to the nervous 

 system as in Balanoglossus. 

 They both differ from Balano- 

 glossus in having the aliment- 

 ary canal bent on itself so that 

 the anal opening is situated not 

 far from the mouth ; in the 

 presence of 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 Rhabclopleura such openings are entirely absent. Both 

 forms occur in associations or colonies secreting a common case 

 or investment. Both occur at considerable depths in the sea. 



Cephalodiscus has an investment (Fig. 669) in the form of a 



FIG. 669. Cephalodiscus. Gelatinous 

 investment. (After Mclntosh.) 



