MOLLUSCS. 



97 



Fig. 89. — A section through an eye (/) and 

 the large («) and small (b) sense-organs of 

 chiton. The lens of the eye is shown at y, 

 and the retina at n. 



account of the peculiarities of their structure and growth. One featun: 

 must be mentioned. For years chitons had been studied, and every text- 

 book stated them to be blind until, in 1884, Professor Moseley stated that 

 some species were not blind, but that they had hundreds, and even thou- 

 sands, of eyes scattered all over the back, 

 and along with them were other thou- 

 sands of sense-organs, also unknown be- 

 fore. With the growth of the animal 

 the eyes also grow old, and the calca- 

 reous cornea becomes scratched and worn 

 away, so that the optic apparatus is of 

 but slight value ; but at the same time 

 new eyes are being developed at the free 

 edge of the shell, so that the loss of the 

 old is made good by the new. The chi- 

 tons are all marine ; some are small, 

 others reach the length of a foot. On 

 our east coast the species are few and 



small, but on the shores of California and farther north they are very 

 abundant, both in species and individuals. 



The naked molluscs, the so-called Nudibranchs, are among the most 

 beautiful animals in the sea. They are very abundant, and yet they are 

 rarely seen except by the experienced collector. The reason for this is 

 their wonderful protective coloration. On the green fronds of the sea- 

 lettuce (Ulva) sharp eyes will occasionally detect a slug-like animal, as 

 green as the plant on which it lives, while in the parti-colored hydroids 

 occur naked molluscs whose hues and tints agree exactly with their sur- 

 roundings. In a group where this protective resemblance is so well 

 developed, it is hard to say what species show it best ; one example 

 seems to serve almost as well as another, but possibly the forms known as 

 Scyllcea and Dendronotus are the most striking. 



Scyllcea occurs on the gulf-weed which forms such banks in the tropi- 

 cal Atlantic. In its shape it resembles a bit of the gulf-weed, and the 

 similarity is heightened by the lobes which project from the body. The 

 color, too, is the same, and until the animal leaves the plant one but 

 rarely is conscious of his presence. Even when it loosens its hold and 

 sinks slowly through the water, it takes sharp eyes to recognize the differ- 

 ence between it and a bit of the sea-weed. The other form, Dendronotus, 

 is more northern in its distribution, occurring: on our shores as well as 

 on those of Europe. Its general appearance is shown in our figure 

 and it can readily be seen how easily the animal can escape observation 



