258 LEPTODORA HYALINA. 



Daphnia is that the carapace, which in Daphnia is so large as to enclose 

 the entire body like a bivalve shell, is so much reduced in Leptodora as to 

 present only a small incubating chamber in the female, only partially 

 covering the body, and not more than one-fourth of its length ; and in 

 the male this appendage almost vanishes, and is represented by a mere 

 rudimentary projection. The first segment of the general integument, 

 namely, the head covering, instead of lying close down, and terminating 

 in a pointed beak as in Daphnia, is prolonged upwards in Leptodora, as 

 a tapering pedestal, on the top of which is placed the large single eye. 

 From the next segment of the integument, namely, the covering of the 

 thorax, there is developed the enclosing casing of the incubating chamber, 

 which is the same in structure and origin as the large enclosing casing of 

 Daphnia, being a fold of the chitinous or horny integument which is 

 doubled in upon itself, forming a double hollow wall to enclose the 

 incubating chamber, as shown by the dotted line in Fig. 3. The eggs, 

 when in this chamber, are entirely outside of the body, being between two 

 walls, both of which are external surfaces of the general integument. 

 In Daphnia this double fold is extended so far as to cover the entire body, 

 but in the male Leptodora the fold is shortened so much as to become 

 only a short projecting pouch, not covering the body, as seen in Fig. 1. 



The swimming arms, or posterior antennae, are greatly enlarged and 

 prolonged in Leptodora, and have a remarkable development of powerful 

 muscles. The extreme transparency of the object allows the form and 

 attachments of these muscles to be seen very clearly, and with polarised 

 light they present a singularly beautiful view. 



The eye is a large spherical group of lenses, as shown enlarged in Fig. 7, 

 Plate X. It is seated directly upon the brain mass, from the lower portion 

 of which two nerves branch laterally to the anterior antennas, and a pair 

 of diverging nerve cords pass down to ganglia situated immediately above 

 the mouth. The anterior antennas, or " antennules," are very short in the 

 female, terminating abruptly with a fringe of knobbed hairs, but in the 

 male thesa antennas are long and tapering to a point. The single eye of 

 Daphnia and other Entomostraca was considered by Swammerdam (as 

 quoted by Baird) to be a pair of ordinary hemispherical eyes united into 

 one, but this view was not entertained by subsequent authorities. From 

 the structure of the eye in Leptodora there however appears good reason 

 to conclude that Swammerdam's view was correct ; and it will be seen 

 on consideration that such a view has a strong support from analogy. 

 The eye in Leptodora is divided into two lateral halves by an interval 

 at the base, where it is seated upon the brain, and also by a gap left open 

 at the top, but the division at the top does not show in most specimens, 

 and the impression is consequently given that, when the division is 

 shown, as in the specimen from which Fig. 7 was drawn, it is a case of 

 incomplete development, reverting to an earlier type of separate eyes. 



The eye has a partial rotation round its centre, as in the case of 

 Daphnia, and the three muscles shown on each side of the eye serve to 

 give this motion. The general structure of the eye, as represented in 

 the elementary diagram, (Fig. 8,) consists of a set of radiating conical 



