26 



(illustr. on pi. II to pi. VII etc.). In all species with well-developed head, or at least with 

 the frame left, we see behind the frontal margin and inside the lateral parts of the frame 

 a broad band of thin, soft skin. Somewhat behind the middle of the frontal margin is the 

 rostrum (proboscis) with antennae and maxillulae, and from this part backward towards, or 

 quite up to the list behind the basis of the maxillipeds, we see a system of plates or lists. 

 This system, which I shall call the nub-median skeleton, is partly or all the way divided 

 into two halves by softer skin along the median line; its structure differs in nearly every 

 species; as a rule it expands considerably in the middle of its lateral margins. The inner 

 margin of the basal joint of the maxillae touches the outer margin of the front part of the 

 expansion, whereas the maxillipeds are articulated behind the expansion touching the outer 

 margin of the narrower posterior part of the skeleton. 



In several species of the genus Sphceronella, namely Sphcer. microcephala, S. dispar, 

 S. insignis, S. Munnopsidis and S. marginata (pi. VIII, fig. 2d; pi. IX, fig. 3 f and fig. 4c; 

 pi. X, fig. 4b, and pi. XIII, fig. 6d), there is no separate head and no harder chitinous borders 

 (only in 8. marginata and in S. microcephala there is a low border or a transverse list in 

 front of the mouth (pi. XIII, fig. 6d, pi. VIII, fig. 2e)), whereas the sub-median skeleton exists, 

 strongly developed as a solid plate in S. Munnopsidis (pi. X, fig. 4b), much reduced in S. dispar 

 and S. insignis (pi. IX), and particularly so in S. marginata. 



The Antennulce. In all species, except the five without separate head and without 

 frame, the antennulae are well developed, and in these they are articulated to the solid frame, 

 each at one of the angular points where the lateral margin merges into the frontal margin (comp. 

 e. g. pi. II, fig. In and fig. 3a, pi. XI, fig. la). Each antennula is usually composed of three 

 joints, of which the second is generally the shortest, the third the longest. The front angle 

 of the first joint is mostly provided with two or three shorter or longer setae; the terminal 

 joint is rather well provided with bristles of different length, among which an olfactory seta 

 (b) can be frequently pointed out. In the genus Homoeoscelis the antennulae become 2-jointed 

 by the fusion of the second and third joints (pi. XIII, fig. Id). In Sphcer. decorata (pi. VIII, 

 fig. 3e) the first and second joints are coalescent. In Sphcer. marginata the antennulae 

 (pi. XIII, fig. 6 d) are constructed as in Homoeoscelis. In the other four species of Sphceronella, 

 which are devoid of separate head and of frame, the antennulae are situated at the same 

 points, but fastened to the thin membrane, besides being shorter and reduced so as to show 

 only indistinctly separated joints or no division at all. 



The Antennae. These organs I have been unable to discover in the species of the 

 genus Homoeoscelis, and in Sphcer. modesta, S. dispar, S. insignis, S. marginata, S. Munnopsidis 

 and S. microcephala, whereas they exist in the other species of Sphceronella and in Chonio- 

 stoma. They are always placed on the side of the rostrum itself near the margin of its 

 expanded basal part, and they are always short, slender, generally 3-jointed (e. g. pi. V, fig. 2 d), 

 without hairs and terminating in one shorter or longer seta. In a few species, e. g. Sphcer. 

 decorata, the number of joints is reduced to two, in Sphcer. antillensis (pi. Ill, fig. 2 c) they are 



