12 bulletin: museum of comparative zoology. 



la. ha.) rests as a cap on the dorsal surface of the ventral cup. It 

 may be regarded as a single structure, though it is perforated by open- 

 ings, through which the nerves pass from the retinal cells (Plate 5, 

 fig. 54); la. ha. and fhr.). The plate is oval in outline, but does not 

 extend to either the anterior or posterior border of the ventral eye 

 (Figs. 3, 6), nor does it quite reach the lateral margins of that portion 

 of the eye (Fig. 7). The plate of the ventral eye is in all respects 

 similar to those of the lateral eyes, except for the openings in it already 

 mentioned. The description of the latter may be deferred until the 

 discussion of the innervation of the eye is taken up. 



It should be added that, although in entire preparations the basal 

 plates of the eyes seem to be very thin shells, they are in reality of 

 considerable thickness. In Figure 49 (Plate 5) the plate as shown in 

 the right lateral ocellus {ocl. d.x.) has been cut almost perpendicularly 

 to its two broad surfaces. In other figures where the basal plates 

 appear the plane of the section is such that a true idea of the real thick- 

 ness of the plate is not given. There are two reasons for this: first, 

 it is very difficult in embedding the animals to orient them so that the 

 plane of sectioning will be more than approximately that which is 

 desired; and, secondly, on accoiuit of the curvature of the plates, 

 especially those of the lateral eyes, very few if any of the sections coin- 

 cide with the radius of curvature. In Plate 5, Figure 45, the basal 

 plate of the ventral eye is cut almost perpendicularly, and I believe 

 that this gives a fairly accurate idea of the real thickness of this plate. 



We are justified in summarizing the knowledge of the tripartite eye 

 of Eucalanus thus far gained as follows. The paired latero-dorsal 

 and the single median-ventral optic cups rest upon, or in, a single cell, 

 which forms the pigmented background of the retinal elements. In 

 this respect the eye of Eucalanus differs from that of Cyclops or 

 Diaptomus, as described by Hartog ('88) and Richard ('91), where 

 at least more than a single cell exists in the central mass of the eye. 

 In Eucalanus, as in the fresh-water forms, the faces of the central 

 mass (or cell) which are turned toward the optic cups form the 

 tapetal layer; these are to be looked upon as products of the cen- 

 tral cell and not as independent cells. The so-called pigment-plates, 

 which Grenadier ('79) described in the eye of Eucalanus, are proba- 

 bly not such, if we may rely upon comparative evidence adduced from 

 the conditions seen in other Copepoda. But whether Grenadier's 

 interpretation is the correct one or not is of less real importance than 

 the fact that pigment is known to occur in the eye. Its location need 

 not necessarily be known for a fair understanding of the morphology 



