MUSEUM OF COMPARATIVE ZOOLOGY. 49 



tails of the development of this organ were not followed on account of 

 the continual increase of pigment. Bullar ('79, pp. 513, 514) in a paper 

 on parasitic Isopods described the development of the retina in Cymothoa. 

 His account is substantially as follows. In the course of the develop- 

 ment of the cerebral ganglion, wheu this structure is separated from the 

 superficial ectoderm, the latter remains on the exterior of the embryo as 

 a layer of considerable thickness. From this superficial layer is devel- 

 oped the retina, i. e. all parts of the eye which in the adult lie between 

 the basement membrane and the corneal cuticula. 



I have studied a few stages in the development of the eyes in Idotea 

 robusta. The retina in this species originates as a simple thickening in 

 the superficial ectoderm, in essentially the same manner as Bullar has 

 observed in Cymothoa. 



The retina in Isopods, both in respect to its method of development 

 and its general structure in the adult, is unquestionably a representative 

 of what I have called the first type of retinal structure. 



Nebalice. — In Xebalia, as the figures given by Claus ('88, Taf. X. 

 Figs. 8 and 17) show, the retina and adjoining hypodermis are directly 

 continuous, and the former presents all the characteristics of a simple 

 thickening in the hypodermis. 



Stomaiopoda. — In an adult specimen of Gonodactylus which I ex- 

 amined, the relation between retina and hypodermis was the same as in 

 Nebalia. 



Nothing is known, I believe, of the development of the retina in either 

 the Xebalia? or the Stomatopods. The structure of the eyes in the adults 

 of both groups, however, shows very conclusively that their retinas belong 

 to the same structural type as those of Branchipus. 



Schizopoda. — In describing the development of Mysis chamelio, Nus- 

 baum ('87, pp. 171-185) states that the retina arises from a thickening 

 in the superficial ectoderm, and adds that its formation, so far as his 

 observations extended, was not complicated by an involution. 



In Mysis stenolepis, a Schizopod whose eyes I have studied, the 

 retina and hypodermis in the adult are directly continuous, as in Bran- 

 chipus. This relation is what would be expected from the method of 

 development described by Xusbaum. 



Decapod'/. — Carriere ('85, p. 1G9). in his account of the eyes in Asta- 

 cus, showed very clearly that in the adult the retina and hypodermis 

 formed a continuous layer. This relation was subsequently observed by 

 me in Horaarus (Parker, '90 a , p. 5), and I have since seen the same con- 

 dition in Gelasimus, Cardisoma, Cancer, Hippa, Palinurus, Pagurus, 

 vol. xxi. — xo 2. 4 



