68 BULLETIN OF THE 
the ommatidia in other Crustaceans. The order in which the groups 
will be considered is one which is intended to emphasize their relations 
only in so far as the structure of their ommatidia is concerned. Natu- 
rally, this order will vary somewhat from the one usually given in sys- 
tematic treatises. I shall begin with the Amphipods. 
Amphipoda. 
Within recent years the more important types of eyes in the Amphi- 
pods have been studied with such care that the structure of their om- 
matidia is perhaps better known than that of any other large group of 
Crustaceans. My own observations do little more than confirm the 
accounts already published. 
‘The species of Amphipods whose eyes I have examined are Gammarus 
ornatus, M. Edw., Talorchestia longicornis, Say, and an undetermined 
species of Caprella. Of these the specimens of Gammarus and Caprella 
were collected at Nahant, Mass., where I also obtained several sets of 
eggs representing stages in the development of the former. Examples 
of Talorchestia were kindly supplied me from the collections in the 
Museum. 
The corneal hypodermis in Amphipods was first satisfactorily described 
by Claus (’79, p. 131) in his account of the eyes in Phronima. It is 
represented in this genus by a layer of undifferentiated cells lying be- 
tween the corneal cuticula and the membrane which limits the distal 
ends of the cone cells. A corneal hypodermis similar to that in Phro- 
nima has likewise been described by Mayer (’82, p. 122) in Caprella and 
Protella, by Carriére (’85, p. 156) in Gammarus, by Claus (’87, p. 15) in 
the Platyscelide, by Della Valle (’88, p. 94) in the Ampeliscide, and 
by Watase (90, p. 295) in Talorchestia. I have also identified this struc- 
ture in Gammarus, Caprella, and Talorchestia. re 
In Gammarus, as Carriére (’85, p. 156, Fig. 121) has clearly shown, 
the corneal hypodermis at the edges of the retina is directly continuous 
with the general hypodermis. According to my own observations this 
condition is not only met with in Gammarus, but also in Caprella and 
Talorchestia. 
In Phronima, according to Claus’s figures (°79, Taf. VI. Figs. 48 and 
49, Ma Z.), the arrangement of the cells in the corneal hypodermis 
bears no definite relation to the subjacent cones; the distal end of each 
cone presents an area which is covered by about « dozen hypodermal 
cells. In Gammarus I have observed (Plate I. Figs. 2 and 3) an essen- 
tially similar distribution of the hypodermal cells ; as in Phronima, the 
