ON THE STEUOTUBE OF ENIGMA .ENIGMATIOA. 279 



Pallial Organs. — The lobes of the mantle were so much 

 contracted in spirit that I have been unable to make out the 

 details of the histology of the marginal tentacles to my satis- 

 faction. The tentacles are covered by a high columnar 

 epithelium, and receive an abundant nerve supply from the 

 circumpallial nerve. There are some specially large tentacles 

 on the lower posterior edge of the right pallial lobe (fig. 13, 

 t.), and near the bases of souie of these there are ganglionic 

 enlargements on the circumpallial nerve. The thickeued 

 edges of the mantle present the three reduplications com- 

 monly occurring in the Lamellibranchia, and in the bottom 

 of the groove formed by the two outermost reduplications 

 there is, in the hinder part of both mantle lobes, a tract of very 

 definite columnar cells supplied with twigs from the circum- 

 pallial nerve. These cells are probably sensory in function. 



As may be seen in the sections, figs. 9 — 13, the lower 

 moieties of the mantle lobes overlapping the gills ai"e very 

 much thickened, and their inner surfaces are thrown into a 

 number of folds, which are specially prominent in the right 

 lobe. These foldings are no doubt exaggerated by contrac- 

 tion in spirit, but there can be little doubt that they exist in 

 the fresh state, and serve to retain water when the animal is 

 uncovered for long periods by the tide. The thickened lobes 

 of the mantle are nearly entirely composed of a lacunar 

 tissue, traversed by very few branching muscle-fibres. The 

 greater part of the musculature of the mantle lies imme- 

 diately below the external layer of epithelium, and all within 

 this is a spongy lacunar tissue, which must serve as a reservoir 

 for the retention of fluid. 



In this connection it may be observed that the asymmetrical 

 arrangement of the organs, and in particular the narrow 

 attachments of the mantle lobes to the visceral mass except 

 in the regions of the attachments of the muscles to the valves 

 of the shell, induces a certain laxity and independence of the 

 organs, which can be better understood by an examination of 

 figures 11 — 13 than by any description. As a consequence 

 there are numerous irregular spaces communicating with the 



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