532 DR S. F. HARMER AND DR W. G. RIDEWOOD ON THE 



use them, as a rule, as a ready means of distinguishing one species of Cephalodiscus 

 from another. It is remarkable that what one would suppose to be the uncorrelated 

 efforts of the numerous zooids of a colony do in fact produce so uniform a result. 

 The comparison of larger series of specimens may perhaps show in the future that 

 the coenoecial characters are less reliable than they appear to be at present ; but the 

 fact remains that the common house of the colony has, as a rule, so distinct a character 

 of its own that it is difficult to believe that it cannot be used for systematic purposes. 

 It has thus been possible to distinguish the following subgenera of Cephalodiscus : — 



(1) Demiothecia, Ridewood (07^), in which the coenoecial cavity is continuous and the 

 zooids occur separately or in groups in any part of it, being free to wander about in it. 



(2) Idiothecia, Ridewood (07^), in which each zooid, with a certain number of its 

 buds, occupies an independent tube-like cavity in the coenoecium. 



(3) Orthoecus, Andersson (07), in which each zooid has a tube of its own, but 

 the tubes are free for the greater part of their length, instead of being embedded in 

 the common coenoecial mass as in Idiothecia. 



The descriptions which have been published by the observers who have been 

 referred to above show that there is a singular uniformity of structure in the zooid 

 throughout the genus. In such fundamental characters as the three divisions of 

 the body, and their associated ccelomic cavities, the notochord, the proboscis-canals, 

 the collar-canals, and the gill-slits, there is practically no variation. The remarkable 

 character of the male C. sibogse, as described by Harmer (05), indicates that in that 

 species at least there is a striking sexual dimorphism. In those other species in 

 which both sexes are known there appears to be no essential difference between 

 the two sexes except as regards their gonads. An exception must, however, be made 

 in the case of C. inaequatus (probably = C. hodgsoni), where most of the female zooids 

 have five pairs of arms, while most of the males have six pairs (see pp. 560-562). 



Making use of the coenoecial characters which have been mentioned above, it 

 appears to be possible to distribute the known species of Cephalodiscus among the 

 three subgenera there indicated, and in this way to separate species which it might 

 be hard to characterise from the structure of their zooids. But within a single sub- 

 genus it becomes necessary to rely more largely on the characters of the zooid ; and 

 the discrimination of species on anatomical grounds may offer considerable difficulty. 



In the earlier accounts of species of Cephalodiscus some stress was laid on the 

 proportions of the zooid and of its stalk. This procedure has been criticised by 

 Andersson (07), who has had the unique opportunity of examining certain species in 

 the living state. When account is taken of the highly muscular character of the 

 zooid, and of the evidences of contraction afforded by wrinkling of the skin of 

 the stalk and other parts, it is easy to believe that Andersson's criticism is to a large 

 extent well founded. It may, nevertheless, be true that some weight may be ascribed 

 in certain cases, as in discriminating between zooids in a similar degree of contraction, 

 to features of this kind. 



