MUSEUM OF COMPARATIVE ZOOLOGY. 59 



scribed a pair of thickenings, which extend from the superficial ectoderm 

 of the antero-hiteral part of the head to the brain. These thickenings 

 are present only in the early stages of development, and represent the 

 unsevered connection between the brain and the superficial ectoderm. 

 They closely resemble the developing lateral eyes of Cranchipus, and 

 Grobben has therefore very justly considered them rudiments of the 

 lateral eyes. If the rudiments of the lateral eyes in Cetochilus de- 

 velop from the superficial ectoderm, it is probable that the lateral eyes 

 in other Copepods have a similar origin. 



To which of the three retinal types already described the eyes in 

 Copepods belong is not easily decided. The absence of any indication 

 of an optic pocket, either in the development of what Grobben con- 

 siders the rudiments of the lateral eyes in Cetochilus, or in the fully 

 formed eyes in other genera, seems to me to preclude the possibility of 

 these eyes belonging to what I have described as the second type. 

 The separation of the retina from the hypodermis prevents them from 

 being classed with the first type, and, especially in the case of the 

 Branchiura, brings them into close relation with the third type. It is 

 my opinion, that, if the lateral eyes in Copepods are not representatives 

 of a fourth type, essentially different from the three already described, 

 they must be considered members of the third retinal type. 



Certain species of Cumacete, Ostracods, and Cirripeds possess optic 

 organs which probably represent the compound eyes of other Crusta- 

 ceans ; but so far as I am aware, the relation of these structures to the 

 hypodermis is unknown. It is therefore impossible to state whether 

 those eyes represent other retinal types, or belong to one of the three 

 already described. 



According to the preceding account, three retinal types can be dis- 

 tinguished in the compound eyes of Crustaceans. In the first of these 

 the retina is a simple thickening in the supei-ficial ectoderm (hypo- 

 dermis). This type is characteristic of the ej'es in Isopods, the Bran- 

 chiopodidse, the Xebalise, Stomatopods, Schizopods, and Decapods. In 

 the Isopods, the eyes are sessile ; in the other groups of the first type, 

 they are borne on the distal ends of movable optic stalks. 



In the second type, although the retina, as in the first type, originates 

 as a thickening in the superficial ectoderm, it ultimately becomes en- 

 closed within an optic pocket. This may remain permanently open, as 

 in the Apusidse and Estheridee, oi it may become closed, as in the 

 Cladocera. In the Apusidee, so far as I am aware, the eyes are not 



