96 OPISTHOBRANCHIATA OF BRAZIL 



relations of some of these nerves in both the Brazilian and the 

 Neapolitan forms. 



In studying both species the central nervous system was 

 carefully freed from its attachments, stained in Paracarmine, and 

 cleared in glycerine. To facilitate microscopic examination, and to 

 avoid unnecessary displacement of the parts concerned, a piece 

 of glass rod was drawn out in a flame to the approximate diameter 

 of the esophagus of the animal. A short bit of this was passed 

 through the circumesophageal loop of the central nervous system, 

 and the whole mounted on a slide having a suitable depression 

 ground in it. The preparation may then be rotated and examined 

 from all sides, without danger of disturbing the relations of its 

 parts. The use of the Zeiss binocular dissecting microscope 

 enormously facilitates the recognition of the relations of the 

 ganglia and their nerves. 



The dorsal portion of the nerve collar is made up of the 

 fused cerebro-pleural ganglia (PI. XIX, fig. 96, c. pi. g.), in con- 

 tact along the median line, the cerebral portions being joined by 

 the very short and broad cerebral commissure. The two ganglia 

 are marked off from each other by a slight transverse constriction, 

 dividing the complex into two approximately equal portions, 

 the anterior, cerebral one being slightly larger than the posterior, 

 pleural part. While in the main the nerve cells correspond in 

 their distribution to these divisions, there is no middle region 

 between the two entirely free from them. The length of the 

 whole complex is 0.6 mm., the transverse diameter of each cere- 

 bral portion 0.345 mm., that of the pleural portion 0.315 mm. 



Lateral to the esophagus are situated the ellipsoidal pedal 

 ganglia (PI. XIX, fig. 96, ped. g.), connected to the cerebro-pleural 

 complex by the very short cerebro-pedal and pleuro-pedal con- 

 nectives. The maximum length of the pedal ganglia is 0.33 mm., 

 with an antero-posterior diameter of 0.21 mm., and a dorso- 

 ventral diameter of 0.225 mm. 



The ventral portion of the circumesophageal ring is made 

 up of the commissures. These are three in number, two of which 

 are united together in a common sheath, forming a broad band. 

 In this band are included the broad pedal and the narrower sub- 

 cerebral commissures (PI. XIX, fig. 96, ped. com.). Separate 

 from this, but close to it is the slightly longer pleural commissure 



