The anatomy of a pteropod, Corolla (Cymbuliopsis) spectabllis Dall. 75 



one of the lesser branches of this system may be seen to course 

 above the semicircular casque muscle and unite with a branch of the 

 extensive system developed from the larger visceral nerve now to be 

 described. 



The single visceral ganglion, wedged between the pleural ganglia, 

 is slightly asymmetrical in position (Fig. 5). From its posterior bor- 

 ders two nerves arise, the larger {v'^) located almost in the mid line, 

 the smaller on the right side close to the pleural ganglion. The larger 

 visceral nerve (Fig. 9) appears to be distributed wholly to the sur- 

 face of the shell. Like the posterior branch of the left palliai nerve just 

 described it passes backward and dorsally, crosses the semicircular 

 casque muscle and breaks up into a number of fine branches that 

 may be also be traced far around on the ventral surface of the casque. 

 As mentioned previously one of the smaller more median nerves forms 

 an anastomosis with a similar branch from the right palliai nerve. 

 Before reaching the semicircular muscle branches arise that extend 

 outward over the surface of the shell (F'ig, 9). The smaller palliai 

 nerve (v^) usually passes outward a short distance to the left but 

 soon returns to the median line and resting against the under sur- 

 face of the oesophagus passes into the visceral mass (Fig. 8). Di- 

 recting its course to the right it gives oô' a small nerve that is dis- 

 tributed to the area about the external reproductive opening. Farther 

 backward it sends an exceedingly delicate fibre into the gonad, and 

 finally in the region of the heart breaks up into three or four branches. 

 One, sometimes two of these, may be distinctly followed for a short 

 distance upon the surface of the stomach. Another passes up to the 

 pericardial wall and is there lost as is another that appears to pass 

 directly into the visceral mass beneath the gonad. 



Although cephalic eyes are absent there are multitudes of sense 

 organs situated along the margin of the fin that appear to be rudi- 

 mentary light percipient organs. Each in its fully developed con- 

 dition (Fig. 16) consists of what probably functions as a simple lens 

 (I.e.) and retina (r.c). The lens consists of an irregular vesicle filled 

 in life with a fluid of considerable density judging from its refraction 

 and with fragments of cells whose origin will be considered presently. 

 The retina is composed of from five to twenty relatively large cells 

 heavily charged with pigment. In every case a nerve cord, passing 

 beneath this part of the ocellus, sends otf delicate fibres whose desti- 

 nation cannot be accurately determined owing to the large quantities 



