296 Mr. A. Hancock on the Anatomy oj Oithona. 



envelope of the eggs, is composed of two lateral lobes separated 

 on the upper surface by a deep fissure. These lobes are semi- 

 pellucid and are formed of a coarsely convoluted tube ; that on 

 the right side having its anterior portion {n') opakc and flesh- 

 coloured. The two lobes open into the female channel (/), which 

 is wide and much longer than usual. 



The reproductive apparatus, we see, is formed on the type of 

 that of Eolis. The mucus-gland is exactly of the same form, and 

 the mode of union of the androgynous parts with the oviduct and 

 testis is the same as in that genus. The only modification of 

 any interest is in connexion with the testis. We know of no 

 other species, in the whole of the Eolididce, in which it is furnished 

 with a distinct vas deferens. In this respect our new genus re- 

 sembles some of the Derides, particularly Doris repanda, in which 

 the testis has appended to it not only a vas deferens but is like- 

 wise supplied with a much-constricted duct, which unites it to 

 the oviduct just as in Oithona ; and the testis, too, of this Doris is 

 very similar to the same organ in this animal. 



In Oithona, then, as in all the Nudibranchs, it is evident, fi*om 

 the connexion of the various parts of the genitalia, that self-im- 

 pregnation is not only possible but probable ; while at the same 

 time it is apparent that the ova may be fertilized by the congress 

 of two individuals. 



Vascular and Resjnratory Systems. — The vascular system is 

 very interesting in this animal, inasmuch as we have been able to 

 trace the efierent or branchio-cardiac vessels more completely than 

 in any other member of the family. Indeed nearly the whole of 

 these vessels are distinctly visible on the skin of the back, rising 

 above the general surface, and exhibiting a very curious and novel 

 appearance. The heart (PI. IX. fig. 2 c) is situated about the 

 middle of the back, where it forms a large oval swelling imme- 

 diately below the skin, having the generative organs beneath. 

 From the posterior extremity of the swelling a broad elevated but 

 rounded ridge {d) passes down the median line of the back to 

 the termination of the body. This ridge is joined on either side 

 by numerous similarly elevated branches (e), which divide and 

 subdivide as they approach the pallial-like expansion on the sides 

 of the body. The whole of these branches and their subdivisions, 

 standing boldly up from the general surface of the skin, have the 

 branchial papillae set along them (fig. 3 a), and they give off 

 twigs, which pass up the margin of the broad, flounced, mem- 

 branous expansion {b) of the papillae. 



On opening the heart from above, the ventricle and auricle are 

 found to occupy a well-defined oval pericardium. The ventricle 

 (PI. X. fig. 3 a) is large and muscular, of an irregular ellip- 

 tical form, giving off" the aorta (h) in front, which in the usual 



