1893.] MB. A. E. SHIPLEY ON THE GENUS SIPUNCtrLITS. 327 



those of the Common Earthworm, and their presence gives the 

 Si2nmcuhis a remarkable resemblance to that animal. The rings 

 correspond with elevations of the cutis extending over several 

 bundles of the circular muscles which lie just under the cutis. 

 On looking at the skin with a hand-lens it is at once seen that 

 each ring is composed of a number of rectangular oblong areas 

 (Plate XXTI. fig. 4) side by side. Each of these oblong areas corre- 

 sponds in width with one of the bundles of longitudinal muscles 

 which lie within the circular muscles. 



The mouth of Sqmncvlus indicus is situated in the centre of the 

 anterior end of the body ; it is slightly elongated transversely 

 (Plate XXVI. fig. 4). Around the mouth lies a circular ring of ten- 

 tacles, and between the mouth and the ring of tentacles a number 

 (7 or 8) of ridges radiate ; probably these correspond with certain 

 vascular spaces which supply the tentacles. 



Viewed from in front the mouth and tentacles are very much 

 like those of Stepliunostoma (PJiascoIosoma) Jianseni as figured by 

 Danielssen and Keren (4) in theii- monograph on the northern 

 Gephyi-ea. 



Owing to the fact that Sipunculids usually die with their in- 

 trovert inverted, the arrangement of the parts of their head has 

 been difficult to make out, and viith the exception of the figure 

 given by H. B. "Ward (3) it does not seem to me that this part of 

 the body has been adequately depicted. The genus Sijnmculus 

 differs from many other unarmed Gephyrea in having a ciliated 

 web or membrane round the mouth, which web has not been 

 broken up into discrete and independent tentacles. The arrange- 

 ment of the web is very various even amongst members of the 

 same species. The simplest form is that presented by Sipuncuhis 

 tcssellatus (Plate XXVI. fig.7), in ^vhich the web cannot be said to 

 be broken up into tentacles at all, although its free border is very 

 irregular ; it forms a complete ring around the mouth and is not 

 incurved at any point. The inner surface of the web bears 

 numerous ridges and intervening depressions which are lined by 

 ciliated cells. In the specimen depicted in Plate XXVI. fig. 7, some 

 of these ridges were especially marked and continued some way 

 into the mouth. In Sijnmcidus indicus the membrane has been 

 much more broken up into tentacles, which appear more or less 

 aggregated into tufts, and there seems to be a certain relation be- 

 tween the tufts and the conspicuous radial ridges which run 

 toward the mouth. In two of the four specimens which I received 

 from Zanzibar the head was extended : one of them is depicted in 

 Plate XXVI. fig. 4, and it will be noticed in this one that the tentacles 

 are rather thicker on one side, the dorsal, than on the other; in 

 the second specimen with extended head the tentacles were even 

 more concentrated in this region, and showed a tendency to be 

 incurved, so that the outUne of their base took the form of a 

 double horse-shoe. 



In Sipunculus nudus the membrane has not broken up into 

 tentacles, but remains as a web with a ciliated internal surface and 



