SOME POINTS IN THE ANATOMY OF POLYCBJETA. 243 



tissue of the gonad is, in fact, continuous with the posterior 

 angle of the nephrostome. 



There is not, then, a great deal to be added to Cosmovici's 

 account, but there are one or two corrections to be made, and 

 a re-examination of the subject was necessary because the false- 

 ness of his interpretation causes his description to be received 

 with doubt. His account of the position of the nephridia is 

 inaccurate ; they are situated in somites v — x (the fourth to 

 ninth chaetigerous) inclusive. The first or buccal somite of 

 Arenicola is destitute of bristles : the following six somites bear 

 each a dorsal fascicle of capillary chaetae, and a ventral torus 

 uncinigerus, but no branchiae ; the next thirteen somites bear 

 each both fascicle and torus, and, in addition, a pair of plumose 

 branchiae ; the rest of the body, which is of different lengths in 

 different individuals, is thinner, and devoid of fascicle, torus, 

 and branchia ; it is cylindrical and covered uniformly with 

 papillae. Behind the first four somites are incomplete septa. 

 The nephrostome of the first nephridium is on the anterior 

 face of the fourth septum. Between any two successive para- 

 podia, behind the third, are five constrictions, of which the fifth 

 is the deepest. The septum, when present, is attached to the 

 body wall opposite the second constriction. Between the fifth 

 constriction and the parapodium is a prominent ridge ending 

 in a sharp edge. The first nephridia of the first pair are some- 

 what smaller than the rest. The relation of the gonad to the 

 nephridium is shown in PI. XVII, fig. ], which gives a view of 

 the internal face of the nephridium. In the natural position the 

 nephridia are covered dorsally by the bands of oblique muscles, 

 which pass from the sides of the nerve-cord to the line of dorsal 

 bristles ; and in this condition the dorsal lip of the nephro- 

 stome is horizontal, and its edge is directed downwards and 

 inwards. 



Cosmovici says he searched in vain for a long time for the 

 genital organs, and discovered the ovary by accident when 

 examining a piece of the nephridium. I discovered the ovary by 

 tracing the origin of the ova in the body cavity. In February last 

 I found two or three specimens which had ova in the body cavity; 



