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 substanfially as follows. In the course of the develop- 
ment of the cerebral ganglion, when 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. 
Nebalie. —In Nebalia, as the figures given by Claus (88, Taf. Xx: 
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. 
Reaiennde: — In an adult specimen of Gonodactylus which I ex- 
amined, the relation peaneen retina and hypodermis was the same as in 
Nebalia. 
Nothing is known, I believe, of the development of the retina in either 
the Nebaliz 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 Nusbaum. 
Decapoda, — Carriére (’85, p. 169), 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 Homarus (Parker, ’90*, p. 5), and I have since seen the same con- 
dition in Gelasimus, Cardisoma, Cancer, Hippa, Palinurus, Pagurus, 
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