498 HEMICHORDA 



that t)f Annelids ; and the gill-slits are unrepresented there, 

 though Salensky has described oesophageal pockets opening to 

 the exterior in four Annelid types — Polygordius, Saccocirrus, 

 Spio /iiliginosiis, and Polydora cornuta. In the last there are 

 five pairs in the larva, and two persist. If there be a 

 relationship between Enteropneusta and Annelids, it must 

 be a very distant one, perhaps restricted to origin from some 

 common stock. 



Class Pterobranchia. (i) Cephalodiscus 



Cephalodiscus dodecalophus was dredged by the Challenger in the 

 Magellan Straits. Others are known from Japan, the Malay Archi- 

 pelago, South Africa, and the Antarctic. It was at first described by 

 M'Intosh as a divergent Polyzoon, but the researches of Harmer point 

 to relationship with Balanoglossits. 



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 tentacles. These two characters, formerly 

 supposed to indicate Polyzoan affinities, may perhaps be adaptations to 

 the sedentary life. With Balanoglossus this type has been compared, 

 on account of the possession 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 ; {h) each of the three regions contains a coelomic cavity, 

 the most anterior being single, while the other two are divided by a 

 median partition ; (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 diverti- 

 culum from the gut, which extends towards the proboscis region ; this 

 has been compared to the " notochord " of Balanoglossus ; (g) the 

 anterior region of the gut is perforated by a pair of lateral gill-slits. 

 The gonads lie between anus and pharynx. Buds are given off from 

 a lateral stalk. 



(2) Rhabdopleura 



This genus is found at considerable depths in the North Sea and 

 Atlantic. Like Cephalodiscus, the individuals are minute and stalked, 

 and occur in a colony ; in this case, however, they remain attached to 

 one another by a common stolon, instead of being united only by an 

 investment. The proboscis or buccal shield makes a thin annulated 

 tube within which the polyp moves up and down. In the head region 

 there are two hollow lateral arms bearing numerous ciliated tentacles, 

 which have a skeletal support. The gut, as in Cephalodiscus, has a 

 U-shaped curvature and an anterior diverticulum (" notochord "). 

 There are five coelomic cavities, and two collar-pores. There are no 

 gill-slits. 



