ECHIUROIDEA. 533 



The resemblances between the Sipunculoidea and Echiuroidea have been 

 referred to above. They formerly led to the union of the two groups in the 

 class Gephyrea, and they consist mainly in the resemblance between the brown 

 tubes of the former and the anterior nephridia of the latter, in the spacious 

 character of the coelom in the two groups, and in the singleness of Jthe ventral 

 nerve cord. As to these we may remark that nephridia tend to resemble each 

 other in groups much more remote from each other than those under discussion, 

 that a spacious coelom is found in many widely divergent forms, and that the 

 singleness of the ventral cord, though an important feature, is not of itself 

 sufficient to outweigh the following most important differences: (1) the difference 

 in the position of the anus ; (2) the absence of a preoral lobe in the Sipuncu- 

 loidea ; (3) the absence of anal vesicles in the same group ; (4) the absence of 

 any traces of Annelidan setae ; and (5) the total absence of any trace of 

 segmentation in the larva and in the adult. 



Echiurus Cuv. (Fig. 428). Proboscis not bifurcated ; anal setae in one or two 

 rows ; two pairs of anterior nephridia ; males and females alike. E. pallasii 

 Guerin, N. Sea, English Channel, etc. 



Thalassema Gaert. Proboscis not bifurcated ; no anal setae ; one to four pairs 

 of anterior nephridia ; sexes alike. Th. neptuni Gaert., English Channel, etc. 



Bonellia Rolando (Fig. 429). The animals are of a green colour, owing to the 

 presence of a pigment distinct from chlorophyll, and called bonellein. Proboscis 

 bifurcated ; no anal setae ; a single anterior nephridium ; sexual dimorphism 

 pronounced. B. viridis Rolando, North Sea, etc. 



Eamingia Dan. and Kor. Proboscis not bifurcated ; no ventral hooks and 

 no anal setae ; one or two anterior nephridia ; sexual dimorphism pronounced ; 

 males provided with ventral hooks. H. arctica D. and K. 



Saccosoma Dan. and Kor. Proboscis absent ; no hooks or setae ; one anterior 

 nephridium ; male unknown. S. vitreum D. and K. 



Epithetosoma Dan. and Kor. , may be placed here, though it differs considerably 

 from other Echiuroidea. The proboscis is long and whip-like, and contains a 

 cavity continuous with the body-cavity. Posterior to this tubular proboscis, 

 on either side of the anterior extremity of the trunk, is a fissure, the bottom of 

 which is pierced with apertures leading into the body-cavity. There are no 

 hooks or setae, and no anal vesicles. The mouth is at the base of the proboscis, 

 and the anus is posterior ; the alimentary canal is straight. The nerve-cord 

 contains a tube. Vascular system not observed. 



