GEPHYREA 163 



cells. From the longitudinal bundles four stout muscles arise, 

 two dorsal and two ventral. These pass to form a muscular 

 ring ensheathing the oesophagus, just behind the head. They 

 are termed the four retractors, and their function is to draw 

 in the introvert. 



The alimentary canal consists of a straight oesophagus, 

 into which the mouth passes without any armature, and 

 which in its turn passes into a coiled intestine. Both these 

 parts are ciliated, the ciha of the oesophagus being continuous 

 with those of the lower lip and tentacles. The intestine is 

 coiled round a special " spindle " muscle, which arises from the 

 extreme posterior end of the body, passes up the axis of the 

 coil, and joins the longitudinal muscles of the body-wall near 

 the anus (Fig. 104). A short rectum passes to the anus 

 which terminates the alimentary canal, the anus pierces the 

 body-waU just behind the line of division between the introvert 

 and the trunk. 



The vascular system is closed and is confined to the anterior 

 end of the animal. Its most conspicuous part is a vessel which 

 lies on the dorsal side of the oesophagus between the retractor 

 muscles. The vessel is closed behind, and gives off no capillaries. 

 At the anterior end it opens into a large sinus into which the 

 brain protrudes ; from this sinus a circular vessel is given off 

 which runs round the lower lip, and when fuU of blood, it 

 serves to distend the latter. Another part of the vessel runs 

 along the base of the lophophore, giving off branches into each 

 tentacle. It is possible that the blood may become oxygenated 

 in the tentacles, but the chief function of the whole system is 

 to distend the tentacular crown and lower lip. The fluid in 

 this system is corpusculated. 



The coelom is very spacious, and contains a corpusculated 

 fluid which bathes aU the internal organs. The corpuscles 

 are larger than those of the vascular system. The contraction 

 of the circular muscles of the skin forces this fluid forward, 

 and in this way the introvert is everted. 



The nephridia, or excretory organs, of the Gephyrea are 

 often termed " brown tubes." In Phymosoma they are two in 

 number, one on each side of the ventral nerve cord (Fig. 104). 

 They have the form of elongated sacs, which hang down 



