TETHYS CERVINA DALL AND SIMPSON 55 



wall of the pericardium, bifurcates to the kidney, the dorsal peri- 

 cardial wall and the auricle near the entrance of the branchial vein. 

 In Tethys dactylomela and in the Mediterranean Aplysiidae 

 studied by Mazzarelli ('93, Monog. p. 108) the third and fourth 

 nerves, here described as separate for Tethys cervina, are united 

 in one trunk for some distance from their origin. 



THE REPRODUCTIVE SYSTEM. 



The ovotestis (PI. VI, fig. 33, ov. t.) is an irregular lobulate 

 organ, situated at the posterior end of the visceral mass, closely 

 united to the liver in front, and inclosed in the last turns of the 

 intestine. From its median antero-dorsal face the small herma- 

 phroditic duct (PI. VI, fig. 33, sm. h. d.) arises, a white nearly 

 straight tube, 9.0 mm. in length, gradually increasing in diameter 

 from 0.3 mm. as it emerges from the ovotestis, to 0.8 mm. near the 

 adnexed genital mass. The adnexed genital mass is a flattened, 

 elliptical complex, made up of the nidamental and albumen glands 

 and the fertilization chamber, inclosed in the loops of the genital 

 duct, (fig. 33, sp. p., c. />.), and is situated immediately 

 behind and below the right posterior border of the pericardium. 

 It is 2.0 mm. in length, 1.3 mm. in greatest width, and i.o mm. in 

 thickness. Its position is in almost direct prolongation of the 

 large hermaphroditic duct, which extends forward along the right 

 body wall to the external opening. At the left of its basal end 

 the spermatocyst (PI. VI, fig. 33, sp. c.) projects transversely as 

 a free pear-shaped sack, its length, i.o mm., being one-half the 

 length of the adnexed genital mass, while its diameter is nearly 

 0.5 mm. Its duct (PI. VI, fig. 33, d. C.}, the "duct of Cuvier," 

 opens into the proximal end of the copulatory duct (PI. VI, fig. 33, 

 cop. d.). The stout large hermaphroditic duct is 4.5 mm. in 

 length, one-half that of the small hermaphroditic duct, its diam- 

 eter i.o mm. being practically the same throughout. It is made 

 up of two channels, separated by deep folds of the dorsal and 

 ventral walls, which overlap in the median line thus forming the 

 ovo-spermatic duct (PI. VI, fig. 33, ov. sp. d.} and the copulatory 

 duct (PI. VI, fig. 33, cop. d.}. At its distal end it is slightly 

 enlarged and receives the long slender duct of the vesicle of 

 Swammerdam, the spermatotheca, (PI. VI, fig. 33, spth.}, which 

 enters from the left side and above. This duct is 3.0 mm. long 



