438 F. W. GAMBLE AND J. H. ASHWORTH. 



stomium is similar in the two species (PL 22, fig. 5). In front 

 it is continuous with the papillose margin, forming an upper 

 lip which overhangs the mouth. 



III. Eyes and Nuchal Organ. — There are two struc- 

 tures developed from the prostomium, the eyes (PL 24, fig. 

 34) and the ciliated grooves or nuchal organ, which may be 

 considered under the head of external organs, though the 

 former and the greater part of the latter are hidden when an 

 adult specimen is viewed from without. The structure of 

 the eyes is more fully considered in the section on the sense- 

 organs (p. 506, and PL 27, fig. 55). It will be sufficient to 

 state here that they are comparable with the earliest stage 

 of the development of eyes in Nereis as regards structure, 

 but that they are sunk even in early post-larval stages of A. 

 marina into the ganglionic layer of the brain. In this 

 position they persist apparently throughout life in all species 

 of the genus. 



The nuchal organ or ciliated groove is a V-shaped ciliated 

 but deep and naiTOw groove, formed by an invagination of 

 the epidermis of the sides and hinder end of the prostomium, 

 as is well seen in the figure of A. Claparedii on PL 27, figs. 

 56 and 57, from a specimen 26 mm. long. The arms of the 

 groove diverge, and run somewhat outwards and downwards 

 as far as the anterior cerebral lobes, while their hinder ends 

 are connected by a curved portion lying at a higher level. 

 Since, however, the groove is sunk deeply into the body its 

 appearance in horizontal section is that of a pair of tubes 

 commencing anteriorly in the angle made by the projection 

 of the lateral prostomial lobe, and then running inwards and 

 backwards, so that only the dorsal sections show the union 

 of the two grooves near the hinder edge of the prostomium, 

 although of course they are in connection with the exterior 

 along their whole length. 



In A. cristata the nuchal organ is essentially similar to 

 that of A. marina described in our previous paper, and it 

 is only necessary to emphasise the single character of the 

 organ ; for although it is innervated from a pair of posterior 



