G8 BULLETIN OF THE 



the onimatidia 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 Carriere ('85, p. 15G) in Gammarus, by Claus ('87, p. 15) in 

 the Platyscelidse, by Delia Valle ('88, p. 94) in the Ampeliscidse, and 

 by Watase ('90, p. 295) in Talorchestia. I have also identified this struc- 

 ture in Gammarus, Caprella, and Talorchestia. 



In Gammarus, as Carriere ('85, p. 15G, 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. YI. 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 a 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 



