ANATOMY AND CLASSIFICATION OP THE ARENICOLID^. 439 



cerebral lobes there is no reason for regarding it as a double 

 structure, at least in full-grown Arenicola, though it may 

 arise from a paired origin, and is undoubtedly equivalent to 

 the pair of ciliated eversible organs in Capitellids and 

 Opheliacea, and to the two longitudinal grooves on the 

 prostomium of Maldauidse. In a very young specimen of 

 Arenicola marina in our possession (a post-larva 4*5 mm. 

 long) the nuchal organ is already present, and consists of 

 a median transversely placed depression '04 mm. from side 

 to side and "012 mm. deep. 



The groove is similar in both A. Grub ii and A. ecaudata, 

 while its course differs from that hitherto described (PI. 22, 

 fig. 5). The arms of the groove commence ventro-laterally, 

 and run upwards and backwards, but before meeting on the 

 dorsal surface of the prostomium they bend sharply back- 

 wards and then unite in a short transverse line. In A. 

 ecaudata the thickened lips are nearly black in colour and 

 fairly wide apart, and thus contrast with the pink tint of the 

 bottom of the groove. In A. Grubii the lips are more 

 closely apposed. 



In this section of the genus the nuchal organ is further 

 distinguished by its innervation. In place of the cerebral 

 lobes of A. marina there are several posterior outgrowths of 

 the commissure-like brain, which have rather the appearance 

 of nerves than of lobes. In some specimens, however, a 

 median process of the brain extends backwards so as to 

 underlie the dorsal part of the nuchal groove. The organ is 

 already present in post-larval specimens of A. ecaudata 

 7'2 mm. and 9*4 mm. long. 



IV. Peristomium. — The peristomium is fused with an 

 annulated achtetous region, which lies in front of the first 

 parapodium. The composition of this region is difiicult to 

 determine, but it can be shown to result from the fusion of 

 the peristomium and the first chaetigerous segment, the seta 

 of which is vestigial and disappears early. The grounds for 

 this conclusion rest upon (1) the external delimitation of 

 the peristomium from the first segment in the post-larva of 



