422 SUB-PHYLUM HEMICHORDA. 



Affinities with Echinodernis (as suggested by Metchnikoff). 



(1) The Turnaria has some resemblance to an Echinoderm larva 



(Bipinnaria), e.g. in the longitudinal ciliated band and the 

 dorsal pore. 



(2) In some Echinoderm larvae there seem to be five primitive 



divisions of the ccelom. 



Besides these affinities, others have been ingeniously detected. Those 

 alleged to exist between Enteropneusta and Nemerteans, e.g. the exter- 

 nal ciliation, the unsegmented musculature, the correspondence of the 

 " notochord " and the Nemertean proboscis, the nature of the nervous 

 system, the sacculations of the gut, the arrangement of the gonads, are 

 perhaps even more unsatisfactory than those above cited. 



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. 



Class PTEROBRANCHIA. (i) Cephalodiscus. 



Cephalodiscus dodecalophus was dredged by the Challenger in the 

 Magellan Straits. Others are known from Japan and the Malay 

 Archipelago. 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 in. by 6 in. 

 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 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 ; 

 (b] each of the three regions contains a coelomic cavity, the most 

 anterior being single, while the other two are divided by a median par- 

 tition ; (<:) the anterior pre-oral cavity opens to the exterior by two 

 pores (cf. proboscis pore of Balanoglossus] ; (</) 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 continued to some extent into the proboscis ; 

 (_/) beneath the nervous system lies a diverticulum from the gut, which 

 extends towards the proboscis region ; this has been compared to the 

 " notochord" of Balanoglossus, but, according to Masterman, the com- 

 parison is not justified ; (g) the anterior region of the gut is perforated by 

 a pair of lateral gill-slits. These gill-slits are supported by vacuolated 

 " notochordal " tissue, and beside them lie a pair of blind diverticula of 

 the gut, whose walls are composed of similar cells, which are compared 

 by Masterman to double " notochords. " 



