258 



PROCEEDINGS OF THE ACADEMY OF 



[March, 



a lens or cornea. There is no reason to doubt the correctness of Kowa- 

 levski's observations, but the eye of Chiton polii is certainly not typi- 

 cal. On the other hand, the eyes of the chitons I have examined are 

 in their histological details essentially like those of the annelid trocho- 

 phore. In the latter organism they are placed in the velar field and 

 are innervated by nerves from the cerebral ganglia; in the chitons they 

 are posttrochal and are situated on the pallial cords. These facts, 

 however, may not be fatal to the theory that the ocelli of the larva of 

 these two phyla are homologous, especially in view^ of the fact that their 

 early development is almost identical. I have shown in another paper^ 



A. Section through ocellus and pallial cord of sexually mature Trachydermon 

 raymondi (6 nun. long). B. Anterior part of nervous system of Ischno- 

 chiton mertensii (4 mm. long); h.g., buccal ganglion; sr.g., subradular 

 ganglia; o., ocellus. C. Section through eye-spot of annelid (Sabella) 

 trochophore. 



that the head vesicle, or the part of the chiton larva anterior to the 

 velum, "becomes transformed into part of the first valve of the shell, 

 the mantle and mantle furrow of the same region, and into the pro- 

 boscis." Now it is obvious that if the chiton eye were situated in 

 front of the velum, as in the annelids, it would be most unfavorably 

 placed after the metamorphosis. Under the circumstances the most 

 available situation would be the furrow about the proboscis, where it 



^ "The Development of Ischnochiton," Zool. Jahrb., Bd. XII, 1S99, p. 630. 



