298 Mr. A. Hancock on the Anatomy of Oithona. 



pose are seen in the wall of the great median trunk-vein. Here 

 then, as in Doris, the blood is partly aerated in specialized breathing 

 organs, and partly in the skin. In this respect also our animal 

 resembles Eolis, in which some time ago, we, in conjunction with 

 Dr. Embleton*, described the existence of two canals, passing up 

 the margins of the papillse, much in the same manner as in this 

 animal, and then pointed out the fact as evincing the probabi- 

 lity of a system of veins, such as the anatomy of Oithona has 

 brought to light. 



We have not been able to carry our investigations of the vas- 

 cular system further ; but from what we know, we cannot doubt 

 that the peripheral portion of it is made up of lacunae as is usual 

 in the Mollusca, and probably to the same extent as in the other 

 Eolididce* 



In connexion with the vascular system, Oithona is provided 

 with an additional propelling organ similar to what in Doris we 

 have called a portal heart. As in that genus, this propelling 

 organ (PL X. fig. 1 s, and fig. 3 e) lies below the floor of the 

 pericardium, and in like manner opens into that organ. In this 

 species it is considerably elongated, with the ends rounded, and 

 is placed far back on the right side of the pericardium. It is 

 firmly attached to the skin of the body, and is internally longi- 

 tudinally plicated. Judging from our knowledge of this heart in 

 the Dorides, it may be supposed to throw venous blood from the 

 pericardial cavity to the glands of the papillae. In Oithona it is 

 certainly connected with the skin, and probably has some rela- 

 tionship to the vascular apparatus therein. 



In this genus a renal organ probably exists, though we have 

 failed to detect it. Adistinct small orifice (PI. IX.fig.2 Z>,and PI. X. 

 fig. 1 t), however, opens externally immediately above the anus, 

 and close to the posterior border of the heart. We traced this orifice 

 through the skin, but could not observe its connexion with any 

 internal organ ; yet there can be little doubt that it is of the same 

 nature as the minute opening by the side of the anus in Doris, 

 and which in that genus leads into an extensive renal apparatus. 

 We have not observed a similar orifice in any other species of the 

 Eolididce. 



Nervous system. — The cerebral ganglia resemble those of Doris 

 rather than of Eolis. They are placed at the commencement of 

 the gullet ; there are as usual four pairs of supra-cesophageal 

 ganglions, though at first sight only three are apparent, — the 

 cerebroid (PI. X. fig. 4 a, a) and branchial (b, b) being completely 

 fused. These latter form two oval central masses, resting upon 

 the upper surface of the gullet, one on each side of the median 



* Ann. Nat. Hist., 2nd Ser., vol. i. p. 101. 



