INTERNAL ANATOMY 255 



made for the new species C. kempt. With the incomplete knowledge of the Umited 

 number of species, it would be futile to search for a primitive type among them. The 

 zooids of all recorded species, except for differences in the number of arms, size of body 

 and position of stolon, are uniform in structure except C. densiis, in which the structure 

 of the gill region marks a distinct advance on the general type. The coenoecium of 

 Cephalodiscus is marked by great diversity of form and structure upon which the classi- 

 fication into sub-genera is based. 



In Orthoeciis the colonies are in the form of cake-like masses, each zooid occupying 

 a tube of its own. The different tubes are not continuous but are held together by the 

 secretion of coenoecial substance between them. In one species — C. futnosus — the 

 individual tubes are arranged vertically inside a common mass which is formed by the 

 agglutination of minute particles of sand. This species and the diminutive disc-like 

 colonies of C. indiciis can be regarded as the bases upon which a hypothetical primitive 

 type can be founded. Such a form will be a solitary individual which constructs a tube 

 of its own, buried in the sand or mud of the sea-bottom, with habits like those of the 

 allied group Enteropneusta. 



The sub-genus Orthoeciis is an intermediate stage between the primitive solitary 

 hypothetical type and the typical colonial forms included in Demiothecia and Acoelothecia. 

 In fact Orthoeciis is only a gregarious group of organisms, each individual having an 

 independent existence of its own. The tendency towards complete colonial life is more 

 definite in Idiothecia where the colonies grow vertically. Though in C. nigrescens each 

 individual has a tube of its own, the elaborate branching of the colonies and other habits 

 are the evidence of a more communal life. This is accentuated in C. agglutinans, in 

 which the different tubes are connected with each other resulting in a tubular labyrinth 

 in the middle of the branch and this may be regarded as the bridge between Idiothecia 

 and Demiothecia. Complete colonial life is met with in Demiothecia and Acoelothecia. 

 The spaces inside the colony are continuous and the zooids occupy them in common, 

 but of these two, Acoelothecia is more advanced, since the central tubular cavities are 

 replaced by the elaborate meshwork which in Form B of C. kempi reaches great size 

 and thickness. 



In the following classification the diagnoses of the species not recorded in this report 

 are adopted from Ridewood. 



CEPHALODISCIDA 



Genus Cephalodiscus, M'Intosh 



Sub-genus Orthoecus, Andersson 



Colonies in the form of a cake or cone or mass of irregular shape. Zooids occupying 

 individual tubes, each with an ostium of its own, embedded in a common coenoecial 

 substance or mass formed by the agglutination of minute particles. The tubes either 

 closely set and parallel or irregularly bent and straggling. 



