44 8 GEPHYREA 



features, with the possible exception of the bristles on the laryi 

 Phascolosoma. On the other hand, it must be remembered that 

 the development of Sipunculus is remarkably abbreviated, and 

 that such stages may have dropped out, the larvae hardly differing 

 more from the Trochosphere of Echiurus and Thalassemia than 

 does that of Bonellia, an undoubted Echiurid. Still the facts 

 that there is never a head-kidney present, that there is no trace 

 of segmentation, and that at no stage is the anus terminal, must 

 have a certain weight. 



If we leave out of account the larval history, which, although 

 pointing to a difference in the nature of the two families, is by 

 no means decisive, and consider the adult structures, we find 

 very considerable evidences of affinity. Taking firstly the main 

 points of difference, we find these to be (i.) the nature of the 

 cephalic appendages, either a proboscis or some modification of 

 tentacles ; (ii.) the position of the anus ; (iii.) the presence of 

 anal vesicles ; (iv.) the number of the nephridia, never more than 

 one pair in Sipunculids ; and (v.) the difference in origin of 

 the chaetae. Of these most undoubtedly the first is the most 

 important. The Echiuroidea have retained the prae-oral lobe of 

 the larva in the form of a solid outgrowth of the body, which 

 outgrowth has- carried with it the nerve-ring and vascular ring 

 which surround the mouth. This has been lost in the Sipun- 

 culoidea, but is, I think, represented by a modified patch of 

 epidermis which lies dorsal to the mouth and just above the 

 brain. A solid extension of the skin in this region, which 

 involved the nervous and vascular systems, would bring about 

 the same relation of parts as is found in the Echiuroidea. 

 The tentacular membrane or tentacles of the Sipunculoidea have 

 such a variety of form and arrangement, whilst all subserving 

 the same end, that I am inclined to believe that they have origi- 

 nated within the limits of the family. 



The position of the anus in the Sipunculoidea is one common 

 to very many animals which live embedded in sand or in 

 tubular holes ; it is probably not primitive, as in the larva of 

 Sipunculus it is near the posterior end, and becomes more dorsal 

 as the larva elongates. 



The anal vesicles of the Echiuroidea probably have no repre- 

 sentative in the Sipunculoidea. In appearance and position they 

 are very like the little tufts which are found on the rectum of 



