53 



inconspicuous mesentery from the posterior wall. It has been possible in this species, as in C. 

 dodecalophus^ to trace the vascular loop from the bend of the alimentary canal (PI. Ill, fig. 22) 

 down the posterior side of the stalk, while the anterior vessel of the stalk is continuous with 

 the vessel in the ventral mesentery of the body. The .stalk-vessels in this species have been 

 observed to contain a distinct coagulum, confirming the view that they are really parts of the 

 vascular system. 



The stalk of C. sibogae (fig. 136) resembles that of C. gracilis. The muscular layer is 

 composed of specially strong fibres, and the amount of connective tissue is small. The nerve- 

 tract of the anterior side forms a prominent ridge. Remains of the mesentery can sometimes 

 he distinguished on the anterior and posterior walls of the stalk, but the state of preservation 

 is often too unsatisfactory to allow of the certain identification of the stalk-vessels. 



A considerable number of stalks of C. sibogae end proximally in a single basal disc, 

 one of which is shewn in vertical section in PI. VIII, fig. 94. The individuals thus connected 

 have presumably arisen by budding from a single parent zooid. A similar arrangement is found 

 in C. gracilis and is described below, in connexion with the buds (Section XVI). 



XI. ALIMENTARY CANAL. 



The alimentary canal of all the species of Cephalodiscus is constructed on essentially 

 similar lines. Its general features may most easily be studied in sagittal sections (PI. IV). 



If the orientation which is adopted in this Report be correct, the whole of the outer 

 outline of the 6''-shaped tube may be described as ventral, and the whole of the inner outline 

 as dorsal. 



The size of the lumen of the several parts of the canal differs greatly in different 

 individuals, the differences being due partly to age, and partly to the condition of mu.scular 

 contraction of the body. One of the parts which shews the greatest variety of form is the 

 mouth, the appearance of which is closely correlated with the attitude assumed by the proboscis. 



In C. dodecaloplms (fig. 42) the mouth (;«.) is indicated by a rather sudden thickening 

 of the epithelium on both its dorsal and ventral sides, giving rise to distinct upper and lower 

 lips. The upper Ifp («. /.) is continuous with the thin ventral epidermis of the proboscis-stalk, 

 and it is in close relation with the posterior ventral horn of the anterior body-cavity. The lower 

 lip passes into the inner epidermic layer of the operculum, a recess [pp. rec.) being left between 

 that organ and the lip when the operculum occupies the position shewn in fig. 42. This recess 

 may be a conspicuous feature of sections through the oral region of some the species. 



Immediately behind the upper lip is a conspicuous dorsal diverticulum (div.) of the 

 pharynx [ph.). This diverticulum comes into close contact with the posterior end of the central 

 nervous system {c. 71. s), and it forms a part of the division between the second and third body- 

 cavities. From its anterior side originates the notochord («r//.), a tubular organ which probably 



