20 BULLETIN OF THE LABORATORIES 



.57 mm., the antennules .32 mm. The form is that of a nauphus 

 toward the end of its first phase. The antennules are much like 

 those of males of Moina, being curved abruptly near the middle. 

 They are clothed with a spiny larval integument, which disappears in 

 the next stage. The sensory ganglia in the end and the connecting 

 nerves are present; of the latter there seem to be two bundles having 

 a different course. The larval eye in the middle of the head is dis- 

 tinct and has two lenses, or, rather, crystalline bodies. 



The antennas are of the form usual to larvae of this family, the 

 rami being unequal, the shorter being very indistinctly two-jointed, the 

 longer eighteen-jointed. Near the base a small prominence bears long 

 spines, later to serve a temporary purpose in bringing food to the 

 mouth. The mandiblar palp is indistinctly 6-jointed, the inner ramus 

 of this limb, or mandible proper, has a single spine. 



The thoracic segments are already indicated and rudimentary limbs 

 lie under the larval skin. The abdomen bears two styles, and has a 

 set of muscles adapted to produce anal respiration in the rectum. The 

 stomach is simple and glandular. Although no heart could be distin- 

 guished, blood corpuscles crowd the antennae and other parts of the 

 body, (see figure 4.) Rudiments of the compound eyes are seen on 

 the sides of the head where pigment is collected. 



In the next stage the animal may be .98 mm. long (Fig. 2.) and 

 several changes appear. A well marked scutum covers the mandib- 

 lar and maxillary segments. The antennules have lost their spiny 

 covering and the proportions of the antennae have changed. At the 

 base of the antennae certain organs develop, which present great re- 

 semblances to the branchial sacs of the other feet, but which become 

 the shell-glands of the adult. This is parallel to the like origin of 

 these organs in copepods, as we have demonstrated in Diaptomus. In 

 Limnetes it was impossible to follow the development of the shell- 

 gland. (See figure 6, <?, shell gland; M, mouth; L, labrum ; Md, 

 mandible; Mx, maxilla; Mx 2, second maxilla.) The brain lobes 

 or supra-oesophagal ganglia resemble those organs in cladocera, the 

 optic-lobes being apparently hollow, however. The posterior part of 

 the body is now considerably elongated. The segments of the thorax 

 seem to be all differentiated at once and the segmentation is obscured 

 by a false segmenting of the posterior part of that region or the appa- 

 rent absence of segments. The region about the rectum is open and 

 crossed by the muscles giving it motion. It frequently seemed to me 



