1885.] 



CLADOCERA. 



37 



the longest. It expands somewhat posteriorty, and is there 

 very obliquely truncate, with the upper angle obtuse, the lower 

 corner rather produced and evenly rounded off. The dorsal 

 margin forms, along with that of the head, an uninterrupted 

 and gentle curve, whereas the ventral edges of the valves are 

 comparatively straight, and constitute a somewhat projecting 

 corner on joining the anterior edges. 



The surface of the shell appears quite smooth, witliout the 

 slightest trace of any striæ or other sculpturing, and the inferior 

 edges of the valves are, as usual, fringed throughout their whole 

 length with rather long and dense cilia. The shell-gland (see 

 Pl. 8, fig. 1) is comparatively simple in structure, forming merely 

 a single short loop just behind the mandibles, its upper extremity 

 reaching to about the place where the heart occurs located. 



The eye (see Pl. 7, figs. 1, 2 ; Pl. 8, fig. 1) is comparatively 

 very small, with only a few, rather minute refracting corpuscles 

 in its anterior part. 



The eye-spot, or ocellus, (ibid.), on the other hand, is of 

 quite a remarkable size, being more than three times as large as 

 the eye itself. It is placed a little nearer to the eye than to the 

 apex of the rostrum, and exhibits an irregular-quadrilateral form, 

 the lower side being the broadest and quite straight, whereas 

 the upper is slightly emarginate with the anterior corner pro- 

 duced to an acute point. Viewed from below (Pl. 7, fig. 2). the 

 ocellus appears deeply incised posteriorly, and thus, as it were, 

 consisting of two symmetrical halves. Along the anterior and 

 posterior edges occurs, moreover, a clear, somewhat vaulted border, 

 representing apparently some kind of refracting corpuscles. 



The antennulæ (see Pl. 8, fig. 1) are rather small, reaching, 

 when extended hardly to the tip of the rostrum, and appear 

 to a great extent concealed beneath the anterior part of the 

 fornix. They exhibit a somewhat conical form, tapering. as 

 they do, toward the apex, and are furnished round the edges 

 with small spinules. The apical sensory papillæ are very short, 

 and the tentacular seta is affixed to the outer side at a short 

 distance from the apex. 



