PTEROBRANCHIA. 



441 



The minute individuals are associated 

 together within a gelatinous investment ; 

 the colony may attain a size of 9 in. 

 by 6 in. The gut is curved, the anus 

 being beside the mouth, beneath which 

 are 4-6 pairs of arms with ciliated tent- 

 acles. These two characters, formerly 

 supposed to indicate Polyzoan affinities, 

 may perhaps be adaptations to the sedent- 

 ary life. With Balanoglossus this type has 

 been compared, on account of the possession 



b.s 



FIG. 239. An individual Cephalo- 

 discus. After Ride wood. 



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

 bulging of the body caused by the 

 gonad ; g-a., to the right, bulging of the 

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

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

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

 edge of the buccal shield ; pi. tentacular 

 plumes, 



FIG, 238. Piece of a colony 

 of Cephalodiscus, showing 

 the tubes inhabited by the 

 animals. After Ridewood. 



of the following characters : (a) 

 The body is divided into three 

 regions, which correspond to the 

 proboscis, collar, and trunk of 

 Balanoglossus ; this is especially 

 obvious in the young bud ; (b] 

 each of the three regions contains 

 a ccelomic cavity, the most anterior 

 being single, while the other two 

 are divided by a median par- 

 tition ; (c) the anterior pre-oral 

 cavity opens to the exterior by 

 two pores (cf. proboscis pore 

 of Balanoglossus} ; (d] the collar 

 region is also furnished with two 

 collar-pores ; (e) in the collar 

 region the dorsal nervous system 

 is also placed, and is continued 

 to some extent into the proboscis ; 

 (/") beneath the nervous system 

 lies a diverticulum from the gut, 

 which extends towards the pro- 

 boscis region ; this has been 

 compared to the " notochord " 

 of Balanoglossus ; (g] the anterior 

 region of the gut is perforated by 



