12 



ZOOLOGY 



SECT. 



free-swimming larval stage, takes place in passages in the investment. 

 According to one account the segmentation is complete, but unequal, 

 and a gastrula is formed by invagination : according to another, the 

 segmentation is incomplete, and gastrula is formed by delamination. 

 The larva bears a striking resemblance to that of Ectoproct Polyzoa. 

 Rhabdopleura (Fig. 723) occurs in colonies of zooids organically 

 connected together, and enclosed in, though not in organic con- 



FIG. 723. Rhabdopleura. A, Entire zooid. a, mouth ; b, anus ; c, stalk of zooid ; d, pro- 

 boscis ; e, intestine ; /, anterior region of trunk ; g, one of the tentacles. (After Ray 

 Lankester.) B, Diagram of the organisation : median longitudinal section, seen from the 

 left. a. arm ; an. anal prominence ; col. collar ; col. ne. collar-nerve ; c. s. cardiac sac ; int. 

 intestine ; m. mouth ; ntc. " notochord " : oe. oesophagus ; pr. proboscis ; pr. c. proboscis- 

 coelome ; ret. rectum ; st. stomach ; te. tentacles ; tr. c. trtmk-coelome ; v. n. ventral nerve. 

 (After Schepotieff.) 



tinuity with, a system of branching membranous tubes connected 

 with a creeping stolon. The collar-region bears a pair of hollow 

 arms, each carrying a double row of slender tentacles the whole 

 supported by a system of firm internal (cartilaginous ?) rods. 

 There are two collar-pores, each leading into a ciliated canal with an 

 internal funnel, and a pair of proboscis-pores. The " notochord " 

 and the nervous system resemble , those of Cephalodiscus. The 

 sexes are united. 



