CEPHALODISCUS 493 



new varieties or possible species when they become spread out 

 in isolated groups without ready intercommunication. The case 

 probably illustrates isolation as a factor in evolution. 



Two very peculiar types — Cephalodiscus and Rhabdopleura 

 — have so many characters in common that they are included 

 by many authorities in one order — PTEROBRANCHIA. 



They are minute animals living in tubes and producing 

 numerous buds. The " proboscis " is flattened into a buccal 

 shield ; the " collar " is prolonged into two or more feathery 

 arms ; the food-canal is bent like a U so that the anus lies 

 dorsally near the mouth. 



The individual Cephalodiscus is 2 to 3 millimetres in length 

 and its appearance is not at first sight suggestive of Chordate 

 affinities. (Plate XXXVI., E, F.) In some features, how- 

 ever, it undeniably resembles Enteropneusts. The body may 

 be divided into proboscis, collar, and short trunk. The pro- 

 boscis, flattened into a shield, overhangs the mouth. The 

 collar is prolonged dorsally into 4 to 6 pairs of plume-like 

 arms each bearing very numerous filaments. The trunk is 

 short and globular. 



The four most remarkable characters are the following : 

 (1) Two gill-slits open from the pharynx to the exterior; (2) 

 the chief part of the nervous system lies dorsally in the collar- 

 region ; (3) a short dorsal outgrowth from the gut in the collar- 

 region passes into the proboscis and is comparable to a noto- 

 chord ; (4) there is a body-cavity in the proboscis, a paired 

 body-cavity in the collar-region, and a paired body-cavity in 

 the trunk. Unless these have been misinterpreted they indi- 

 cate Chordate affinities, and it is a remarkable result of careful 

 anatomy and embryology, that this type, which was originally 

 referred to the same class (Polyzoa) as the common sea-mat 

 {Flustm), should turn out to be not so very far removed from 

 the Chordate stock. Unless there were several independent 

 evolutionary approaches to the Chordate organisation, which is 

 not very likely, then the ancestors of Cephalodiscus were on the 

 main line which has led to Man. What would one not give to 

 know what precisely determined the divergence of the ancestors 

 of Cephalodiscus, so that they ended in being almost seaweed- 

 like aggregates instead of going on ? 



Although a large number of individuals of Cephalodiscus 



