XV 



SIPUNCULOIDEA 



421 



ment is the horse -shoe -like row of tentacles which overhang 

 the crescent if or m mouth, as in Phymosoma and some species of 

 Aspidosiphon. 



The ventral side of each tentacle is grooved and ciliated, and 

 the grooves are continued into the ciliated mouth. Their dorsal 

 surface is pigmented, and in the hollow 

 of the horse-shoe lies a deeply pig- 

 mented epithelium covering the brain. 



A blood - vessel courses up each 

 tentacle, and usually two channels 

 return the blood to the vascular ring 

 which surrounds the mouth. In those 

 forms which possess tentacles on the 

 dorsal side of the mouth only, the 

 ventral part of the vascular ring lies 

 in the lower lip, which is tumid and 

 swollen. The brain supplies a nerve 

 to each tentacle. 



When the introvert is retracted 

 the tentacular ring is withdrawn and 

 to some extent collapsed ; in this con- 

 dition it would be almost touching 

 the rough external surface of the 

 introvert. In some species of Phymo- 

 soma the delicate appendages of the 

 head are guarded from the hooks on 

 the introvert by a thin membrane or 

 collar, 1 which completely ensheaths 

 the retracted he v ad. 



When the introvert is fully ex- 

 tended the dorsal blood-vessel con- 

 tracts and sends its blood forward 

 into the vascular ring, and thence 

 into the tentacles or tentacular fold, 

 which are thus erected. In several 



species of Sipunculus, as S. nudus, S. norvegicus, S. robust us, S. 

 tesselatus, there is a ventral blind tube as well as a dorsal, into 

 which the blood is withdrawn when the head is retracted. In 

 many other species in various genera, such as Phymosoma 



1 Shipley, Quart. J. Micr. Sri, vol. xxxii. 1891, p. 111. 



FrG. 214. A, Phymosoma granu- 

 latum F. S. Leuck. x 2. B, 

 Head of the same. x 4. a, 

 Pigmented pit leading to brain. 

 The crescentiform mouth on 

 the lower side of the figure is 

 overhung by the tentacles. . 



