70 



convenient to retain the term , ganglion , as a name for this region of the 

 oesophageal ring. 



The pleural and cerebral ganglia (with a slight exception mentioned below) 

 consist of small cells whose nuclei vary in diameter between 4 and 7 micra and 

 whose cytoplasm is scarcely discernable , the tissue having a granular appearance 

 under a low power of the microscope. A small lobe of the cerebral ganglion , 

 forming the lower part of its front end and surrounding the tracts which pass 

 out into the cerebro-buccal and cerebro-propedal connectives, differs from the rest 

 of the ganglion in that the cells are larger. The nuclei are from 5 to 13 micra 

 in diameter and the diameter of the cytoplasm is twice or three times that of 

 the nucleus. In contrast to these ganglia , all the other ganglia of the squid have 

 a greatei' or less proportion of large cells whose nuclei vary from 18 to 20 micra 

 in diameter and whose cytoplasm has a diametei' two or three times as large as 

 that of the nucleus. 



The optic ganglia have relatively few large cells while all other ganglia have 

 a fair, though varying proportion of them. The difference in the size of the cells 

 is very marked and clearly distinguishes the cerebral and pleural and. in a large 

 measure , the optic ganglia from all other ganglia. 



The nerve cells are external and the fibre tracts internal in all ganglia, though 

 the boundary between them is often very irregular. 



The cerebral ganglion is the largest ganglion of the squid with the exception 

 of the optic ganglia. It fills up the concavity of the cerebral process of the skull. 

 The ganglion is rounded except that a small lobe from which the cerebro-buccal 

 and cerebro-propedal connectives arise , projects forward from its ventral side. No 

 nerves arise from it , it is very closely connected with the so-called pleural 

 "ganglia" thru which it sends fibres to, or receives them from, the optic, pedal, 

 and visceral ganglia. The lower part of the cerebral ganglion is formed partially by 

 a large tract which connects the tw^o optic lobes and appears to have nothing 

 to do with the ganglion thru Avhich it passes. The fibres which form the cerebro- 

 buccal and cerebro-propedal connectives can be traced in the cerebral ganglion 

 some distance. One tract is at first distinguishable near the dorsal end of the 

 ganglion and as it passes forward it joins the other tract -which arises in the 

 neighborhood of the pleural ganglion. The fibres of the tract thus formed, at least 

 partially , cross to the opposite side and as they leave the ganglion the fibres 

 form two bundles, the connectives. 



The pleural ganglion is quite continuous with and inseparable from the 

 cerebral ganglion above and is hardly distinguishable from the pedal and visceral 

 ganglia below. Its inner surface rests against the wall of the oesophageal canal 



