ANATOMY AND OLASSIFIOATION OF THE ARENICOLID^. 551 



X. All species of Arenicola possess eyes, at least during 

 their immature stages of growth. Although these organs 

 have been previously discovered by Mesnil and Fauvel in 

 post-larval A. marina and A. ecaudata (in which they per- 

 sist), their presence in A. Olaparedii, A. cristata, and A. 

 Grubii is here noted for the first time. The eyes arise in 

 the skin of the larva, and gradually assume a deeper position, 

 until in the adult they are found among the ganglionic cells 

 of the brain. They consist of a clear lenticular body par- 

 tially surrounded by a cup-shaped mass of brown or red 

 spherical pigment granules, which apparently are embedded 

 in a single cell. At their earliest appearance they correspond 

 in position to, while thi'oughout life they retain the structure 

 of, the larval eyes of Nereis. Still more closely do they 

 resemble the simple eyes of the Capitellid Mastobranchus. 



XI. The minute structure of the otocystic epithelium of A. 

 cristata resembles that of the other species, but there is a 

 more marked distinction between the supporting and sensory 

 elements. In A. Claparedii we have not found any ti-ace of 

 the otocyst. The structure and microchemical reactions of 

 the otoliths of A. Grubii are precisely similar to those of A. 

 ecaudata, and point to the conclusion that in both cases 

 these bodies are of chitinoid nature. 



XII. There are no traces in the post-larva (i. e. the stages 

 of growth after the full number of segments has been 

 attained, but in which the gills have not yet assumed their 

 adult number and arrangement) of nephridia other than 

 those found in the adult. Nor is there a suppression in the 

 number of these organs in the life history of the animal. But 

 there is one noticeable histological alteration in connection 

 with sexual maturity. Nephrostomes are absent from the 

 nephridia of our post-larval specimens of A. marina 4'5 mm. 

 long, as they were from Benham's specimen G"8 mm. long. 

 They are present in later stages, and in post-larv[ie of A. 

 ecaudata 8 mm. and 9*4 mm. in length. It is probable that 

 the nephrostomes develop independently, and afterwards fuse 

 with that portion of the nephridium already present in the 



