Xl] SIPUNCULUS 171 



eye of the observer are the alimentary canal and four great retractor 

 muscles of the introvert (i, fig. 69). These latter structures are 

 arranged as a dorsal and ventral pair, they broaden out towards 

 their posterior ends and are all inserted in the body-wall at the 

 same distance from the mouth. Immediately behind the line of 

 their insertion a slight thickening of the coelornic epithelium may 

 be observed running round the body; this is the rudiment of the 

 genital organ. From it in periods of genital ripeness the genital 

 cells are produced and budded off into the coelomic cavity and 

 float in the coelomic fluid. The alimentary canal runs back to 

 the posterior end of the body, here .it bends and runs forward to 

 the anus. It is therefore bent in the form of a U, and the two 

 limbs of the U are twisted round each other in a spiral fashion. 

 From the recurrent or ascending limb of the U not very far 

 from the anus there is given off a blind pouch (/, fig. 69). This 

 pouch varies extraordinarily in length in different individuals, 

 sometimes it is a mere vestige, sometimes it is half as long as the 

 body. The walls of the alimentary canal are extremely thin, 

 they can hardly be touched without rupturing them, and yet the 

 canal is crammed with sharp fragments of shells and other debris, 

 and it is extraordinary that these never seem to penetrate it. 

 The thinness of the walls of the alimentary canal are probably 

 in relation to the poor development of the blood-system, because 

 though there are two blood-vessels on the dorsal side and one on 

 the ventral side of the alimentary canal connected by a ring vessel 

 immediately behind the mouth, yet they extend only a short distance 

 backwards and their main function seems to be to expand the 

 "frilled membrane" to which they give off numerous branches. 

 The products of digestion must therefore diffuse directly into the 

 coelomic fluid, and the effective mixing up of this is aided by the 

 so-called " urns." These are cups of ciliated cells which are formed 

 on the coelomic wall where it covers the dorsal blood-vessel and are 

 budded off into the fluid. 



The organs which function both as kidneys and as excretory 

 ducts are a pair of so-called u brown tubes," which lie beneath the 

 alimentary canal and open on the ventral surface some little distance 

 behind the end of the introvert (h, fig. 69). They receive their 

 name from the colour of the pigment (probably excretory) in their 

 walls. They may be described as blind pouches, with however 

 a lateral ciliated opening leading into the body-cavity and situated 

 near the external opening. They much resemble the "anterior 



