XIII 



PHYLUM CHORDATA 



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 cndostyle to be subsequently met with in the 

 Tunicata (p. 17). The collar-cord is formed by the separating off 

 of the deeper ])ortion 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 adjacent ectoderm. 



Constituting the class Pterobranchia are only the two genera 

 Cephalodiscus and Rhahdopleura. These both resemble Balano- 

 glossus in having the 

 body divided into three 

 parts or regions — a pro- 

 boscis with a proboscis- 

 cavity, a collar with a 

 collar-cavity communi- 

 cating with the exterior 

 by a pair of collar-pores, 

 and a trunk with two 

 distinct lateral cavities ; 

 and in the presence of ?ru} 

 a structure resembling 

 a notochord 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 ; in the presence 

 of arms bearing tentacles 

 arising from the collar : 

 and in the comparatively small size of the proboscis. Cephalo- 

 discus, moreover, has only a single pair of apertures which may 

 be regarded as representing the gill-slits ; while in Rhabdoplenra 

 such openings are entirely absent, their places being taken, 

 apparentl}', by a pair of ciliated grooves. Both forms occur 

 in associations or colonies secreting a common case or investment. 

 Both occur in the sea at various depths. 



Cephalodiscus has an investment (Fig. 711) in the form of a 

 branching gelatinous structure, which is beset with numerous 

 short filiform processes, and contains a number of cavities with 

 external openings occupied by zooids. The latter (Fig. 712) are 



vU.7-^ 



Ctrl' 



Fig. 710— Tomaria. Lateral view. Lettering as in 

 Fig. 60" ; in addition, int. intestine ; mo. mouth. 

 (After Spengel.) 



