SO OPISTHOBRANCHIATA OF BRAZIL 



arising from the outer lower margin of the ganglion and is dis- 

 tributed to the middle region of the foot. Upon the left side its 

 origin is slightly more removed from that of the tenth. 



The ninth pedal nerve (p) arises from the ventral portion of 

 the lower anterior face of the ganglion. It is quite slender and 

 forms an anastomosis with a branch of the seventh. Its distri- 

 bution has been given above in connection with that of the latter 

 nerve, and is similar on both sides. 



The tenth, or anterior pedal nerve (10), is a large trunk 

 from the outer lower margin of the ganglion. It doubles forward 

 in four main divisions beneath the pharyngeal bulb and is dis- 

 tributed to the anterior portion of the foot. 



The order, arrangement and distribution of the pedal nerves 

 is the same for the two species of Tethys here studied, but they 

 disagree markedly with the accounts given by other authors, 

 notably Von Ihering ('77) and Mazzarelli ('93), for the Medi- 

 terranean forms. Until I am able to secure material for a de- 

 tailed comparison of all the species concerned I cannot explain 

 this lack of agreement. Von Ihering ('77) describes and figures 

 but six nerves from each ganglion. Mazzarelli ('93) describes 

 and figures seven paired pedal nerves and one unpaired one upon 

 the right side, and two unpaired ones upon the left, their order 

 and distribution not agreeing with the Brazilian forms, while 

 Lacaze-Duthiers ('87) found but six in all. 



Pleural ganglia. The pleural ganglia (pi. g.), are situated 

 just above, and in contact with the upper surface of the pedal 

 ganglia, with which they are connected by the very short pleuro- 

 pedal connectives. They are about one-third the size of the pedal 

 ganglia, and are spheroidal in outline, measuring 0.5 mm. in 

 greatest diameter. They are made up of large conspicuous cells, 

 which give their surface a knobbed appearance. Contrary to the 

 descriptions and figures of Von Ihering ('77), and Mazzarelli 

 ('93), I find that the pleural ganglia give rise to the following 

 nerves. 



Pleural nerves. From the left pleural ganglion arise two 

 nerves. The first (pi. i), is a slender nerve from the superior 

 face of the ganglion. It passes outward and downward, receives 

 an anastomosing branch from the third pedal nerve and ramifies 



