ANATOMY AND CLASSIFICATION OF THE ARENICOLID.E. 437 



The shape of the prostomium of A. marina was described 

 and figured in our previous paper^ and we have only to add 

 that A. cristata has an organ of almost exactly similar 

 shape. In specimens from Jamaica^ Naples,, and the Pacific 

 coast of America it is uniformly of a trilobate shape, and 

 measures about 1 mm. in length by slightly more in breadth. 

 It is hollow^ and contains an extension of the coelom accom- 

 panied by vessels. As in the lugworm, the brain is in close 

 association with the skin, and fills up the greater part of the 

 cavity of the prostomium. 



In A. Claparedii, different specimens of approximately 

 the same and of different lengths exhibit a considerable 

 variation in the shape of the prostomium (PI. 27, figs. 59 — 

 61). Its comparatively large size drew Levinsen^s atten- 

 tion to it, and he demonstrated for the first time the presence 

 of the prostomium as a distinct structure in the Areni- 

 colidte. Some of the Californian specimens show a marked 

 similarity in prostomial form to the typical adult Mediter- 

 ranean ones, while others differ slightly in this respect, but 

 their differences are paralleled by the variation in the shape 

 of this organ in the young Neapolitan specimens as com- 

 pared with adults. Unfortunately we have never seen A. 

 Claparedii alive, and we are unable to state how far these 

 differences may be accounted for by irregular contraction 

 of the prostomial lobes, and how far they may represent 

 natural variations in the form of the organ itself. The 

 form of the organ is essentially similar to that of A. 

 marina, but the median area is often conspicuously raised, 

 while the lateral projections are bent downwards and out- 

 wards, so as almost to suggest in some specimens that they 

 represent traces of tentacles. Though hollow, the anterior 

 lobes of the brain extend into them, so it is not probable 

 that they are of tentacular nature. A comparable pro- 

 stomium occurs in Scalibregma, in which the anterior 

 prostomial margin is produced outwards like the cross of 

 a T (see Rathke, 1843, pi. viii). 



In the "ecaudata" section the conical, non-lobate pro- 



