Presence of Eyes in the Shells of the ChitouidcC. 145 



lines wliich in the intermediate shells separate the central from 

 the lateral areas, and which correspond in position with the 

 marginal slits and the courses of the principal nerves. There 

 are six rows of eyes on the anterior shell, two on each of the 

 intermediate shells, and six on the posterior shell — twenty- 

 four rows altogether, with an average of about fifteen eyes in 

 each, or in all 360 eyes. In the specimen examined all the 

 rows except one have the eyes arranged in a single straight row 

 at regular intervals, but at the base of one row there are, as 

 an exception, two eyes side by side. There are also a very 

 few irregularly scattered eyes on the lateral area, showing that 

 the condition here existing is probably derived from one in 

 which the eyes were more ancestrally diffused. 



In Acanthopleura sjnniger the eyes are irregularly scat- 

 tered around the bases of the tubercles with wliich the surface 

 of tlie tegmentum is covered, and are confined, in the specimens 

 I have examined, to the region of the margins of the shell 

 adjoining the mantle. The eyes in this species seem to be 

 very liable to be broken or to flake off, in consequence of the 

 decay of the surface-laminae of the tegmentum. Hence those 

 remaining on old specimens are tliose probably most recently 

 formed by the mantle at the margin of the tegmentum. In 

 decalcified tegmenta of some species I have seen eyes thus 

 apparently in process of formation and not yet completed. 

 In some specimens to be referred apparently to this species I 

 have been unable to find any eyes at all. It will be necessary 

 to examine a series of specimens of various ages to discover 

 whether the eyes are originally more widely extended over 

 the shell-surface or always marginal only in this species. 



In a large GorepMum aculeatum, the exposed shells of 

 which were densely covered by a green alga, immense numbers 

 of eyes were found when the alga was scrubbed off, and at the 

 newest margin of the shell not yet encroached upon by the plant. 

 The eyes are very small and their corneas are oval in outline, 

 the long axis of the oval being directed vertically parallel 

 with the height of the shell. The two kinds of pores are 

 arranged in vertical parallel lines with great regularity, the 

 large pores occurring at intervals in the lines of smaller pores. 

 The eyes are never placed on the tubercles, with rows of which 

 the shell is covered, and which are possibly contrivances for 

 protecting the eyes from being rubbed and destroyed. 



The eyes are present in enormous numbers. I estimate the 

 numbers present on the anterior shell alone at 3000, counting 

 only the younger ones, which are in good condition, near the 

 free margin of the tegmentum, and not the older eyes, more 

 or less destroyed by the boring of the shell by algte and 

 animals on the rest of the. area. On the remaining shells, at 



