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79 



The second anterior papilla. This is slightly inside the center of 



the anterior surface. 



In addition to these papillae, there is a prominent ridge which is horizontal 

 and lies on the ventral (front) end of the external surface. 



The statocystic cavity is lined by a pavement epithelium except in the 

 region of the macula statica and crista statica. The former is an area of colum- 

 mnar, ciliated, sensory epithelium covering the ventral or front end of the 

 cavity. The statolith, a club shaped concretion rests upon the macula. The 

 nerve fibres arising from the macula form a number of small bundles and these 

 converging , form two large bundles of fibres which pass out thru the cartilage 

 and then unite. The macular nerve thus formed enters the pedal ganglion and 

 curving outward is lost in the ganglion just outside the roots of the siphonal 

 nerve. The fibres of this nerve do not stain differentially and are small so that 

 it is not easy to follow them far into the ganglion. 



The crista statica is a ridge of ciliated epithelium which winds over the 

 walls of the statocyst. This ridge begins at the foot of the first postero-median 

 papilla runs outward across the floor of the cavity to the front end of the 

 external horizontal ridge , passes backward above this ridge between the first and 

 second external papillae (passing beneath the overhanging part of the first) then 

 turns inward across the floor to the back side of the third posterior papilla, 

 again turns and passes upward to the base of the second anterior papilla where 

 it ends. The crista is formed by three or more rows of columnar, ciliated cells 

 ■with supporting cells. The cells of the central row are markedly larger than 

 those of the two or four lateral rows. The distal ends of the lateral cells incline 

 towards the central cells so that a ridge is formed which in section has approxi- 

 mately the outline of an isosceles triangle. The nerve fibres arising from the 

 anterior transverse portion of the crista form a bundle which enters the cartilage 

 near , but separate from , a larger bundle arising from the remainder of the crista. 

 The latter bundle begins at the dorsal (back) end of the crista and, constantly 

 receiving fibres from it , passes along its upper (anterior) edge. The two bundles 

 pass out of the statocystic cavity at its ventre- external angle and, uniting in the 

 cartilage , enter the pedal ganglion. The fibres pass along the outer surface of 

 the ganglion until beyond the roots of the siphonal nerves and then bend inward 

 forming at least a partial chiasma. The fibres have not been traced beyond the 

 chiasma but are lost in the neuropil e of the ganglion. They are traceable so long 

 a distance because they are unusually large and stain differentially with haemalum 

 or haematoxylin. The fact that the macular and eristic nerves are markedly 

 different does not seem to have been noted by other observers. These two nerves 



