SIPUNCULOIDEA. 539 



larva then contains all the essential organs of the young Sipunculus, 

 except the ventral cord and the blood vessels. At a later stage, 

 during the growth of the larva, the ventral nerve cord is developed 

 from the ectoderm, the circle of cilia disappears, the first tentacles 

 sprout out at the edge of the mouth, and the metamorphosis of the 

 free-swimming larva into the creeping young Sipunculus (Fig. 436) 

 is completed. The anus is never posterior, but is dorsal at its first 

 appearance. There is a small preoral lobe with an apical plate in the 

 larva, which disappears in the adult, the mouth becoming terminal. 

 It is important to notice that the anus is on the same side of the 

 body as the preoral lobe, and is therefore truly dorsal. Whether it 

 marks the morphological hind end of the body, as it does in Phoronis, 

 or whether it is, as described in the text, on the anterior part of the 

 true dorsal surface, cannot be certainly determined, though from its 

 position at its first appearance it is reasonable to conclude that it is 

 at least dorsal in position. 



Fam. Sipunculidae. With the characters of the order. 



(a) Longitudinal muscles of the body-wall divided into bands. Physcosoma 

 Sol. (Phymosoma Sel. ), with papillae ; Sipunculus L. , without papillae ; S. nudiis 

 L., English Channel, North Sea. 



(b) Longitudinal muscles continuous. Phascolosoma Leuckart, two nephridia, 

 numerous tentacles; P. vulgare Blaiuv. , English Channel and N. Sea; P. 



elongatum Kef. ; P. papillosum Thompson, E. Channel ; Dendrostoma Grube, 

 two nephridia, 4-6 tentacles ; Phascolion Theel, right nephridium only ; P. 

 strombi Mont., E. Channel. 



(c) A distinct shield at the anus and at the hind end, or a calcareous ring 

 round the anus. Aspidosiphon Grube, with both shields ; Cloeosiphon Grube, 

 a calcareous ring in front of the anus ; Golfingia Lankester, a preanal horny 

 ring, and a posterior horny spike. 



(d) Two foliaceous tentacles. Petalostoma Keferstein ; P. minutum Kef., E. 

 Channel. 



(e) No tentacles or vascular system; one retractor muscle and nephridium. 

 0/ichiiesoma Ivor, and Dan. ; Tylosoma Kor. and Dan. 



