328 



PALLIAL TENTACLES OF LIMA. 



of 8 to 10 centiiiietei's iu the loose broken coral on the inner side of a 

 reef. 



A microscopic examination proved that the internal structiire of the 

 tentacles of this bivalve is even more remarkably like that of the Nautilus 

 cirri than the external structure. A cross sec- 

 tion (fig. 1) shows a large nerve extending 

 through the center of the tentacle; radial 

 muscles pass from the central region to all parts 

 of the j)eriphery: and strong longitudinal mus- 

 cles lie in the spaces between the radial fas- 

 ciculi. The central portion of the nerve is com- 

 posed exclusively of fibers. Upon its surface 

 is an almost continuous layer of nerve cells, but 

 more or less distinct aggregations of nerve cells 

 air found at points corresponding to the an- 

 ntilai' ridges of the surface of the tentacle. 



As in the Nautihts cirri, 

 the epidermal cells are high 

 upon the ridges but very low 

 in the grooves of the ten- 

 tacle (fig. 3). On the outer 

 surface of the tentacle is a 

 narrow longitudinal groove, 

 at the base of which also the 

 epidermal cells are very low 

 (fig. 1). In some cases the 

 epidennal cells were found to 

 be crowded with miicous se- 

 cretion. 



The muscular develop- 

 ment of the Nautilus cirri is 

 greater, and there is more 

 difference in the epiderinis of 

 the inner and outer surfaces 

 than appears in the teiitacles 

 of Lima, but in the charac- 

 teristic structures of the ten- 



FlG. 4 



taeles the differences are 

 those of degree rather than of kind. The striking parallelism of develop- 

 ment of these rather comjDlex structures in forms so widely separated, 

 systematically, is the more interesting when we remember that the ten- 

 tacles of Lima are appendages of the mantle edge, while those of Naii- 

 tihix belong to the head or foot. 



