BALANOGLOSSUS 



497 



(3) 



Ampkioxus. There are, however, 

 many differences in detail — thus 

 the shts open dorsally, not later- 

 ally ; the skeletal bars arc differ- 

 ently disposed ; the blood supply 

 is different. 



" Dorsal nerve-cord." — A dorsal 

 median insinking (Fig. 280, d.n.) 

 of ectoderm, especially strong in 

 the region of the collar, ma}' be 

 compared with the medullary canal 

 of Vertebrates. But it must be 



b.s 





p.s 



(4.) 



Fig. 283. — Piece of a colony 

 of Cephalodiscus, showing 

 the tubes inhabited by the 

 animals. — After Ridewood. 



noticed that there is also a 

 ventral nerve-cord (Fig. 280, 

 v.n.). 



''The coslom.'' — The develop- 

 ment of five enterocoelic 

 pouches is vevy suggestive of 

 affinities with Amphioxus. 



b-^ 



Fig. 284. — An individual Cephalo 



discus. — After Ridewood. 



h., Buds ; st., stolon ; go., to the left, 

 bulging of the body caused by the 

 gonad ; ga., to the right, bulging of the 

 body caused by the stomach ; p.s., pos- 

 terior lobe of buccal shield ; r.l., a red 

 line on the buccal shield ; b.s., dark 

 edge of the buccal shield ; /'/..tentacular 

 plumes. 



32 



Affinities with Annelids (after 

 Spengel). 



The larva (Tornaria) (Fig. 282) may 

 be regarded as a modified 

 Trochosphere, but this points 

 at most to a far-off common 

 stock. Moreover, the neph- 

 ridia,^ usually present in the 

 Trochosphere, are unrepre- 

 sented in the Tornaria. 



The heart lies, as in some An- 

 nelids, dorsal to the gut, not 

 ventral as in Vertebrates ; the 

 dorsal vessel carries blood for- 

 wards, the ventral backwards, 

 as is usual in Annelids. But 

 the double nervous system 

 is essentially different from 



