22 



which the plasticine plates can be prepared. A more striking advantage is that the flexibility 

 of the material makes it posible to rearrange the position of the parts of the completed 

 restoration. One of the great difficulties in deahng with Cephalodiscits is that such organs as 

 the arms, the opercidum, and the proboscis may overlap one another in such a way as to 

 make it impossible to obtain a view suitable for drawing, or in which all the parts can be 

 seen with the necessary clearness. A reconstruction prepared as above indicated overcomes 

 many of these difficulties. The ventral lobe of the proboscis can be turned forwards so as to 

 exposé the operculum, the operculum can be turned down so as to shew the mouth, and the 

 arms can be untwisted or straightened out : — and all this without putting any of these parts 

 into a position which cannot be assumed during the life of the animal. Thus fig. 25 is a 

 reconstruction made from the originals which are shewn in figs. 43 — 53, but it will be seen 

 that the positions of the operculum and of the ventral lobe of the proboscis were modified 

 after the completion of the model. The proboscis was also slightly rearranged in fig. 22, in 

 order to shew the complete outline of the operculum. 



A further advantage of the plasticine method is that a reconstruction made from oblique 

 sections can be cut in a new plane, so as, for instance, to obtain a medlan sagittal section. 



In studying the general anatomy of Cephalodisais, one cannot but be struck with the 

 constancy of the morphological type throughout the genus. The number of tentaculiferous arms 

 indeed appears to depend on the species, but in other respects the organs already known to 

 exist in C. dodecalophus are found, with practically no variation, in all the species — a fact 

 which speaks for the high specialization of the genus. Differences in degree of development 

 are, however, well marked; and indeed the species of Cephalodiscus have well defined characters 

 of their own. But any one of the species would serve as well as any other to give a good 

 idea of the essential generic characters. 



The most fundamental fact in the structure of Cephalodiscus is the division of the body 

 into the three regions, proboscis, collar and metasome; the proboscis containing an unpaired 

 body-cavity, and each of the other two divisions containing its own paired body-cavities. 



The disposition of the three regions is, however, by no means simple, since each of 

 them is specially developed in relation with some plane or axis which does not correspond with 

 the morphological principal axis of the animal. But an initial difficulty in describing the facts 

 is that it is not agreed what is the correct orientation of the Pterobranchia as compared with 

 the Enteropneusta. Cephalodiscits differs from Balanoglossus in the approximation of the anus 

 to the mouth, and in the extension of the alimentary canal into a f/-shaped loop. It can hardly 

 be contested that the region characterised by the central nervous system and the apertures of 

 the oviducts is dorsal ; and it appears to be natural to regard the alimentary canal as extending 

 into an enormous ventral extension of the body, as in PJioronis. If this be the correct orientation, 

 the morphological posterior end of the animal is indicated approximately by the anus. 



This is not the view which is adopted by Masterman (98, 2, p. 513), who considers 

 that "the pedicle is morphologically the hind end of the body", and that the study of the buds 

 favours this interpretation. It appears to me on the contrary that the disposition of the alimentary 

 canal in immature buds indicates that the stalk is a ventral appendage. 



