1904.1 NATURAL SCIENCES OF PHILADELPHIA. 257 



THE LARVAL EYE OF CHITONS. 



BY HAROLD HEATH. 



In Chiton polii, according to Kowalevski, and in Ischnockiton mag- 

 dalenensis, Trachydermon raymondi and Nidtallina thomasi the eyes 

 of the larvae become clearly defined about the time of the first appear- 

 ance of the shell. They are situated immediately behind the velum, 

 half-way up the sides of the body. Concerning their structure Kowa- 

 levski writes/ "They are characterized by the pigment deposited about 

 a central clear body, and are placed almost entirely upon the lateral 

 or branchial nerves." As the figures of this author show, each ocellus 

 at this time consists of a single cell imbedded in the epithelium covering 

 the body. In later stages this condition of affairs in the young of 

 Chiton polii becomes more complex. According to Kowalevski, 

 the pigmented body, with its clear included vesicle, retains its early 

 characters, but migrates "under the skin and upon the branchial 

 nerve. The ectodermic epithelium, situated above the eyes, presents 

 certain modifications which may be related to the function of the 

 ocellus; its cells are here very slender, more elongated than those 

 adjoining, their appearance is different, and it appears to me possible 

 that these cells play a certain role in the transmission of luminous 

 raj'S to the ocellus and perform the function of a cornea." 



While working upon certain problems connected with the larval 

 development of chitons, I have many times noticed the ocelli in 

 several species. In those forms enumerated above, save Chiton polii, 

 I have traced these structures through all stages, from their first 

 development until their final disappearance ; and after the metamor- 

 phosis of the larva have determined their fate in Ischnochiton mertensii, 

 I. regularis, I. cooperi, Katharina tunicata and Tonicella lineata. In 

 its early stages in these species each eye appears essentially as described 

 by Kowalevski, and this state of affairs continues as long as the ocellus 

 may be distinguished. Under no circumstances does it become 

 subepithelial. In many cases the pigmented cell sinks somewhat be- 

 neath the general outer surface of the epithelium and is partially over- 

 arched by neighboring cells, but these are in no wise different from 

 those elsewhere in the skin and never give the impression of forming 



^ " Embryogenie du Chiton polii (Philippi) avec quelques remarques sur le 

 developpement des autres Chitons," .1 nn. Mus. hist. nat. Marseille, T. I, No. 5. 



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