176 BULLETIN OF THE 



the ganglionic mass, and contain a coagulum, but have no pigment. 

 Comparing these two accounts with each other and with that just given 

 of tlie cerebral organ in S. nudus, it will be seen that the tubes lack 

 pigment in S. Gouldii, and that both tubes and pigment are wanting in 

 S. nudus, unless the regions of thickening in the cuticula on the lateral 

 aspect of the cerebral organ noted above be the rudiments of such struc- 

 tures. The optic nature claimed by Shipley for the tubes in Phy- 

 inosoma agrees with their reduction or disappearance in the forms 

 inhabiting the sand. The position of the organs would seem to indicate 

 an homology between the ciliated cushion of S. Gouldii, the deep pit of 

 the preoral lobe in Phymosoma, and the cerebral organ in S. nudus. 

 As to the histological character of the organ in Phymosoma, nothing is 

 found in the account of Shipley. Andrews describes that of S. Gouldii 

 as ciliated and well supplied with nerves. The deep location of the or- 

 gan in S. nudus may be merely for protection, or perhaps due to the 

 development of the glandular area, or even necessitated by the recession 

 of the brain from the surface. The canal is much longer in S. tessela- 

 tus, where the brain also lies deeper in the body, than in S. nudtis. 

 An analogous variation may be seen in the deep-seated lateral organs 

 of the Enopla as compared with those of the Anopla. 



Finally, if it be asked why the whole structure may not be regarded 

 as a degenerate organ, of which the pigmented tubes wei-e originally the 

 active portion, I can only say that the active glandular area and ciliated 

 canal cannot be explained on such an assumption, and still less can the 

 special nervous supply. I studied the structure a long time with this 

 idea in mind, but finally became convinced that it was untenable in 

 every respect. Although the evidence is far from complete, I regard it as 

 an actively functional organ, morphologically the equivalent of the cili- 

 ated cushion of Phascolosomes, and possibly with a more highly specialized 

 function, since it certainly has a more highly differentiated form. 



Such organs are by no means rare. Dewoletzky ('87, p. 277) has 

 given a list of similar ones, and has considered at length their probable 

 function, which he regards as " some sort of general perception as to the 

 chai'acter of the surrounding medium." 



IV. Conclusions. 



If now the account I have given of certain points in the anatomy and 

 histology of S. nudus be compared with that given by Andrews ('90) for 

 S. Gouldii, it will be noticed that, while there is a general similarity, a 



