MUSEUM OF COMPARATIVE ZOOLOGY. 105 



be recognized easily. Directly under the corneal cuticula each pair of 

 hypodermal cells is in contact with similar pairs belonging to adjoining 

 ommatidia, so that the layer here forms a continuous sheet. In a more 

 proximal plane the neighboring pairs of hypodermal cells are not in con- 

 tact (compare Fig. 93, a tangential section in which the extreme right- 

 hand edge represents the condition immediately below the cuticula, while 

 the parts to the left represent central portions successively more proxi- 

 mal in position). The only indication of a separation between the two hy- 

 podermal cells of each pair is seen in the distal projection of the cone 

 between the two hypodermal nuclei (compare Figs. 94 and 96, con.). 



The corneal cuticula in Gouodactylus is facetted, but the proximal and 

 distal faces of the facets are apparently plane. Over the smaller om- 

 matidia the facets are hexagonal in outline, whereas over the larger ones 

 they are rectangular, and their arrangement is often indicative of the 

 tetragonal system. In Squilla mantis, according to Will ('40, p. 7), the 

 facets are hexagonal. 



The cones in Gouodactylus are composed for the most part of a uni- 

 formly granular substance. Distally, they are pointed and probably 

 touch the corneal cuticula ; proximally, they terminate at the rounded 

 end of the rhabdome (Fig. 94). Each cone contains in its distal enlarge- 

 ment four nuclei (Fig. 97, 7il. con.), two of which lie directly proximal 

 to the nuclei of the corneal hypodermis, while the remaining two alter- 

 nate with them (compare Figs. 96 and 97). The proximal part of the 

 cone is divided longitudinally into four segments (Fig. 98). Each seg^ 

 ment, if extended distally, would include one of the four nuclei, and 

 corresponds to one of the four cells by which the cone was produced. 

 In Squilla mantis, according to Steinlin ('68, p. 17), the cone is also 

 composed of four segments. 



The retinular cells of Gonodactylus are of two kinds, proximal and 

 distal. The proximal cells, constituting the retinula itself, surround the 

 rhabdome completely, and extend distally only a short distance beyond 

 it (Fig. 95). They contain only a small amount of pigment, which is 

 concentrated in two regions, at their distal ends and near the basement 

 membrane. The rhabdome is surrounded throughout its length by a 

 thin but rather dense layer of pigment. This layer is more extensive 

 in the smaller ommatidia (Fig. 102) than in the larger ones. The 

 nuclei of the proximal retinular cells (Figs. 94 and 95, nl. px.) are 

 located near their distal ends. 



The number of cells in the retinula of Squilla, as described by Grena- 

 cher ('77, p. 33) and by Hickson ('85, p. 341, Fig. 2), is seven. In 



