CEPHALODISCUS—RHABDOPLEURA. 399 



tory than those above cited. Again, the resemblance between the 

 Tornaria larva and that of Echinoderms is unsatisfactory, because of our 

 relative ignorance of the development of the Tornaria. 



Here, then, we have a lesson in uncertainties, for all that we can say 

 is that the Enteropneusta seem to be synthetic, possibly transitional 

 types, exhibiting affinities with various others, but differing markedly 

 from all. 



Appendix (i) to Enteropneusta — Cephalodiscus. 



A single species {Cephalodiscus dodecalophus) was dredged by the 

 "Challenger" in the Magellan Straits. It was at first described by 

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

 to relationship with Balanoglossus. 



The minute stalked individuals occur associated together in a gelatin- 

 ous investment, the colony may attain a size of 9 inches by 6 inches. 

 The gut is curved, the anus being beside the mouth, beneath which are 

 two rows of ciliated hollow tentacles. These two characters, formerly 

 supposed to indicate Polyzoan affinities, may perhaps be adaptations to 

 the sedentary life. With Balanoglosstis this type agrees in the possession 

 of the following characters : — (a) The body is divided into three regions 

 which correspond to the proboscis, collar, and trunk of Balaiioglossits^ 

 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 partition ; (r) the anterior pre-oral 

 cavity opens to the exterior by two pores (cf proboscis pores of Balano- 

 glossus) ; (d) the collar region is also furnished with two collar pores, 

 which open beneath a fold or operculum developed from the collar ; (e) 

 in the collar region the dorsal nervous system is also placed, and is con- 

 tinued to some extent into the proboscis ; (/) beneath the nervous sys- 

 tem lies a diverticulum from the gut, which extends towards the proboscis 

 region (" notochord ") ; (g) the anterior region of the gut is perforated 

 by a pair of lateral gill slits. 



Appendix (2) to Enteropneusta — 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 ccelomic cavities, of which the unpaired pre-oral part has 

 two pores. There are no gill slits. The affinities between this type and 

 Balanoglossus must still be held doubtful. 



