CEPHALODISCUS. 355 



ciliated bands somewhat like those of Echinoderm larvae, with an apical 

 sensory plate (like that of many Annelid larvoe), and two eye-spots upon 

 this. The Tornaria becomes pelagic, acquires a proboscis, loses its 

 special bands of cilia, and becomes diffusely ciliated, but has not yet 

 a mouth or anus. 



Johannes Miiller ranked the Tornaria larva, whose adult form was 

 not then known, beside the larvas of Echinoderms. The ciliated bands of 

 the Tornaria resemble those of Echinoderm larvae, but this is only a 

 superficial characteristic. The anterior pouch which forms the cavity 

 of the proboscis and communicates with the exterior has also been 

 compared with the beginning of the water-vascular system in Echino- 

 derms, and it is true that in both several independent coelome pouches 

 grow out from the primitive gut. But almost as good or as bad a case 

 might be made of the resemblances between a Tornaria and an Annelid 

 trochosphere ; and probably a better case can be made of the likeness 

 between certain features in the development of Balanoglossus and that 

 of the lancelet. 



Among the species of Balanoglossus are B. minutus (Naples), B. 

 salmoneus (Brittany), B. robinii (Brittany), B. AoeAleri CEnglisii Channel), 

 B. brooksii (North America), and B. kowalevskii (Chesapeake Bay). 

 The last differs from the others in having a relatively long proboscis, no 

 hepatic sacculations, a, simpler branchial skeleton, very short collar 

 fimnels, a larger backward fold of the collar, and a simpler development 

 without a Tornaria larva. 



CEPHALODISCUS. 



This is a unique genus, having marked affinities with 

 Balanoglossus, and is perhaps another prophecy of the more 

 definitely Vertebrate types. 



A single species ( Cephalodiscus dodecalophus) was dredged 

 by the Challenger in the Magellan Straits. Large numbers 

 of minute individuals live in a branching, brownish, sea- 

 weed-like investment, which may measure 9 inches by 6 

 inches. The organism was monographed as a divergent 

 Polyzoon by M'Intosh, but the researches of Harmer 

 forcibly suggest that Cephalodiscus is allied to Balanoglossus. 



Cephalodiscus is said to agree with Balanoglossus in the 

 following characters : — 



(r) Division of the body into proboscis, collar, and 

 trunk ; this being specially obvious in the young 

 bud. 



(2) An unpaired body-cavity in the proboscis, and 

 paired cavities in collar and trunk. 



