MUSEUM OF COMPARATIVE ZOOLOGY. 83 



The distal face of the retina proper in Argulus is bounded by a deli- 

 cate preconal membrane (Figs. 11-13, mb. ])r'con.) and its proximal face 

 is limited by the basement membrane (Figs. 11-13, mb. ba.). 



The most conspicuous objects in the retina are the cones (Fig. 11, 

 eon.), which lie with their distal ends usually somewhat below the 

 preconal membrane (Fig. 13). Each cone, as Claus ('75, p. 256) has 

 observed, is composed of four segments (Fig. 14). The segments corre- 

 spond to cells, and although the cone itself terminates proximally before 

 reaching the rhabdome, the cone cells form an axis free from pigment 

 and extending from the cone to the rhabdome (compare Fig. 12). In 

 depigmented sections the peripheral membranes of the cone cells (Fig. 

 13, mb. pi'ph.) can be distinguished as sharply marked lines which ex- 

 tend from the sides of the cone to the sides of the rhabdome. The 

 intercellular membranes of the cone cells in the region between the cone 

 and rhabdome are apparently marked by thickenings which appear in 

 both longitudinal and transverse sections (compare Figs. 13 and 15). 

 At the distal end of the rhabdome the four cone cells separate, and, after 

 passing partly around the rhabdome, become lost in the adjoining tissue 

 (Fig. 16, cl. con.). I have not been able to discover the nuclei of the 

 cone cells. 



It is difficult to determine the number of cells in the retinula of Argu- 

 lus. Slightly below the proximal end of the rhabdome, the retinula is 

 divided into five distinct pigmented masses (Fig. 17, cl. rtn.'). Since the 

 rhabdome (Fig. 16, rhb.) is composed of five rhabdomeres, it is highly 

 probable that the retinula consists of five cells; but I have not been able 

 to determine with precision the outline and extent of these cells. 



The nuclei which are visible in the retina of Argulus closely resemble 

 one another. They are limited for the most part to two regions (Fig. 

 13), one near the level of the cones, the other near the basement mem- 

 brane. Apparently there are no nuclei immediately below the preconal 

 membrane. Those which are near the cones (Figs. 13, 14, nl. h\lrm.), 

 judging from their arrangement and position, probably represent inter- 

 ommatidial pigment cells. Those near the basement membrane (Fig. 

 13, nl. rtn.') may be the retinular nuclei, as their position seems to indi- 

 cate. For some distance proximal to the basement membrane, nuclei 

 (Fig. 13, nl. WdrmJ) occur among the nerve fibres. Possibly they repre- 

 sent scattered cells in this region, but the strong resemblance which 

 they have to the nuclei on the distal side of the membrane induces mo 

 to believe that they too are retinular nuclei, which, as in the Amphi- 

 pods, have migrated to a position below the basement membrane. 



