46 OPISTHOBRANCHIATA OF BRAZIL 



order of their appearance from in front and above downward 

 and backward. All abbreviations in the following description 

 refer to fig. 34, Plate VII, unless otherwise indicated. 



Cerebral ganglia. The cerebral (cer. g.) ganglia are com- 

 pletely fused together into a large quadrilateral mass, all traces 

 of their primitive separation into right and left moities having 

 disappeared. The nodulated dorsal surface of the mass is highly- 

 arched, the ventral slightly concave. In width the complex 

 measured 1.5 mm., in length 0.9 mm. and in thickness 0.7 mm. 

 A slender, sub-esophageal, cerebral commissure passes below the 

 esophagus, connecting the halves of the ganglionic mass together 

 ventrally. In fig. 34, PI. VII, s. c. com., it is seen inclosed in the 

 same sheath of connective tissue as the pedal commissure and 

 lying at its anterior margin. 



Cerebral nerves. From the cerebral ganglia arise six pairs 

 of nerves and three connectives, the origins of which are the 

 same for each side. 



The first cerebral nerve (c. 7) arises from the anterior outer 

 face of the ganglion, and courses forward to its distribution to 

 the skin and the muscles of the mouth region. 



The second cerebral nerve ^c. 2') arises immediately behind 

 the first, and is much larger. It curves forward along the 

 pharyngeal bulb, giving ofif, midway of its length, a strong outer 

 branch to the anterior tentacle, in which it terminates in a small 

 ganglion. The main trunk breaks up into a number of divisions, 

 which are distributed to the muscles and integument of the mouth 

 region. 



The third cerebral nerve (c. 5) arises from the upper dorsal 

 border of the ganglion, and is distributed mainly to the rhino- 

 phore, in which it terminates in a small ganglion. It also gives 

 off two or three slender branches, the first and largest of which 

 forms, apparently, an anastomosis with the optic nerve. It is 

 not a true fusion, however, the two nerves being merely united 

 in a common, epineural sheath for a short distance. The dis- 

 tribution of this and the other slender branches of the third 

 nerve is to the integument in the neighborhood of the eye and 

 rhinophore. Upon the right side a true anastomosis occurs with 

 a branch of the second pedal nerve, as shown at x. in the figure 

 and to be described further on. 



