52 



a. 0. BARS. 



[No. 1. 



adult animal, tkougk still exhibiting some minor differeuces. 

 The yolk has now become coinpletely absorbed, and the intestiue 

 is filled up with a clear orange-coloured content, apparently 

 derived from some kind of nourishing matter secreted within the 

 incubatory cavity, as shown to be the case with certain Cladocera. 

 The heart (/<) is fully developed and easily observable from 

 its rapid pulsations, and the blood-corpuscles are in part seen 

 flowing through the several divisions of the body in the usual man- 

 ner. The head has nearly assumed the form characteristic of 

 the adult animal, the rostral part being rather expanded, though 

 still without any trace of denticles. The inferior edge of the 

 head in front of the labrum appears deeply sinuated, and the 

 upper edge somewhat irregularly waved, with the occipital notch 

 well marked, though not so deep by far as in the adult animal. 

 The dorsal pivot-shaped process (x) has sunk, so as alm ost to be ob- 

 solete, forming only a circular, and very slightly elevated area. 

 encircled by a thickened opaque zone. The compound eye (0) 

 is about to develop, though not yet distinctly defined, but with 

 traces of botn the crystalline cones and the pigment. Behind 

 it, the optic nerves and the cerebral ganglion are distinctly 

 observable. The antennulæ (a 1 ) have increased considerably in 

 size and retired from the anterior part of the head, so as 

 to project backwards beyond the scape of the antennæ. The latter 

 organs (a 2 ) are also much more fully developed than in the pre- 

 ceding stage, the segmentation of the scape and of the branches 

 being far more distinct, and the spines and setæ being about to 

 appear. The oral parts w r ould seem to be fully developed and 

 are found to be already active in conveying food to the mouth. 

 The trunk is now divided into the full number of segments 

 (sixteen), the two posterior of which are still however very short 

 and less distinctly defined. On the dorsal surface of the poste- 

 rior segments the setiferous processes are about to be formed. 

 The legs have commenced their rhythmical movements and are 

 present in the full number, the two last pairs {p 15 p 16 ) being still 

 however but imperfectly developed, and immobile. The caudal 

 plate (T) has nearly assumed its definitive form, and the caudal 



