552 P. W. GAMBLE AND J. H. ASHWORTH. 



post-larva. Furthei* evidence in favour of this independence 

 between the two parts of the nephridium is afforded by (1) 

 the occasional presence of a funnel alone^ representing the 

 first and last pairs of nephridia of A. marina and the first 

 pair of A. Grubii; (2) the presence of a funnel only on one 

 side behind the last normal nephridium in A. Grubii; (3) 

 the presence of secretory and terminal portions only^ as a 

 variation in A. marina. 



XIII. All the species of Arenicola are of separate sexes. In 

 all, the gonads arise exclusively in close connection with the 

 nephridia. They are formed by proliferation of the peri- 

 toneal covering of a nephrostomial vessel, which Ave call the 

 gonidial vessel. In the structure of the gonad A. ecaudata 

 is widely removed from the other species, and is almost 

 unique among Polychaeta. In this species there are two 

 deeply staining strands of cells, one on each side of each 

 nephrostome. Only one of these strands, the larger one 

 nearer the middle line of the body of the animal, is fertile. 

 These strands occur also on the first pair of nephrostomes, 

 but in this position they never give rise to reproductive cells. 

 In the case of the remaining twelve pairs of fertile gonidial 

 strands, the peritoneal tissue, of which they are proliferations, 

 is continuous in front with the peritoneum of the neck of the 

 funnel, while further back it forms the covering of the 

 gonidial vessel which runs down the side of the secreting 

 portion of the nephridium. From this vessel outgrowths are 

 formed at intervals towards the middle line, each outgrowth 

 carrying with it peritoneal covering of the vessel. All the 

 outgrowths are connected at their bases by the original 

 gonidial vessel and its peritoneum. The ovaries are formed 

 by further division and growth of the peritoneal cells of the 

 vessels. The cells of the ovary are of two kinds; (1) an 

 enveloping layer of flattened cells, and within this (2) the 

 different stages of the ova themselves. The ovary is well 

 supplied with blood by branches from the gonidial vessel and 

 by anastomosing vessels, which ai'e formed later, and connect 

 adjacent branches. 



