PTEROBRANCHIA. 423 



(2) Rhabdopleura. 



This genus is found at considerable depths in the North Sea. 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 stalk, instead of being united only by an investment. 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. 



As already mentioned, Masterman has recently compared the larva of 

 Phoronis with Cephalodiscus ^ and proposed the re-association of the 

 two animals. The points to which he draws attention are the presence 

 in the Actinotrocha of a true coelom, divided, as in the Hemichorda, 

 into three parts. Of these the anterior or pre-oral is said to communicate 

 with the exterior by two pores, comparable to the proboscis pores of 

 Balanoglossus. The second or "collar" (lophophoral) region opens 

 externally by two ciliated nephridia. Further, the nervous system, 

 which consists of a dorsal ganglion with connected nerve-rings and 

 dorsal and ventral nerve-tracts, is compared in detail to that of 

 Cephalodiscus. An upgrowth from the anterior region of the gut is 

 compared with the structure called by Harmer " notochordal " in 

 Cephalodiscus. Finally, two lateral diverticula of the gut formed of 

 vacuolated cells are compared with the diverticula in the neighbourhood 

 of the gill-slits in Cephalodiscus, and with " notochords." A patch of 

 similar cells occurs on the ventral floor of the gut in the Actinotrocha. 



"While many of the points named above are little convincing as 

 regards chordate affinities, they are interesting as suggesting that the old 

 association of Phoronis and Cephalodiscus was correct after all, and that 

 further investigation will make it possible to draw up a natural classifi- 

 cation of these aberrant worm-like types. 



