ANATOMY AND LIFE HISTORY OP MOLLUSCA. 173 



circulation, as will be seen from the explanation presently to be 

 given of another figure. 



With reference to this section, it may be remarked that some- 

 of the strands of the optic nerve end abruptly after passing 

 through the rods and project in fibres from the surface of the 

 retina. These broken ends may have arisen in cutting the 

 section, though that is not the appearance. The whole section 

 gives a good idea of the nature and extent of the blind spot in 

 the dorsal eyes of these small animals. 



In pi. ix., fig, 14, we have a representation of a fragment of 

 the basilar membrane and pigment-coat surrounding one of the 

 dorsal eyes. This is taken from the outside of a large eye with a 

 very thick circle. The larger perforations seem to be passages, 

 for circulating vessels ; the smaller for nerves which proceed 

 outside the eye to the cornea, argentea, &c. One cannot help 

 remarking the strong resemblance this section has to a section of 

 the basilar membrane of the eyes of insects, given by Mr. Hickson 

 in his paper*. "On the Eye and Optic Tract of Insects." His 

 figure has reference to a portion of the basilar membrane of the 

 eye of Agrion bifurcatum, and it shows much more regular 

 perforations : the larger for the tracheal vessels and the smaller 

 ones for the terminal optic fibrils. In the smaller perforations 

 of my figure, the strands of the nerve-fibre can be seen. Moreover 

 this section manifests the nature of the membrane which in 

 section is seen to surround the pigment-coats ; though the latter 

 are so dark and impervious to light that any membrane is difficult 

 to see. Of course it does not correspond with the basilar 

 membrane of insect eyes, whose place is at the base of the 

 ommatidia upon which they rest. The section is additionally- 

 interesting as affording an idea of the great number of nerves, 

 and circulating vessels which supply the outside of the eyes in 

 these dorsal papillae. The section is magnified about 200 diameters^ 

 The prolongation at the upper end of the fragment would seem 

 to imply that the basilar membrane itself is colourless and only- 

 darkened by the pigment-coat. 



Eyes on the Periostraca. — In one form or another an 

 investing membrane outside the shell is very universal throughout 

 the Mollusca. In some it consists of the merest film ; while in 

 others it forms a conspicuous skin membrane, investing the shell 

 and considerably altering its appearance. It is probable that 

 this investiture is intimately connected with sense-organs, because 

 we find such a very elaborate system of bristles and filaments as- 

 must have some connection with the feeling or hearing of the 



* '* On the Eye and Optic Tract of Insects," by Sydney J. Hickson, B. A.„ 

 Quart. Journ. Microseop. Science for 1885, vol. xxv., p. 215, pi. 17, fig. 28. 



