26 
(illustr. on pl. IT to pl. VII ete.). 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 antennze and maxillule, 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 swb-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 maxilla 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 Spheronella, namely Spher. microcephala, S. dispar, 
S. insignis, S. Munnopsidis and S. marginata (pl. VIII, fig. 2d; pl. LX, fig. 3f and fig. 4¢; 
pl. X, fig. 4b, and pl. XIII, fig. Gd), there is no separate head and no harder chitinous borders 
(only in S. marginata and in S. microcephala there is a low border or a transverse list in 
front of the mouth (pl. XIII, fig. 6d, pl. VIII, fig. 2e)), whereas the sub-median skeleton exists, 
strongly developed as a solid plate in S. Munnopsidis (pl. X, fig. 4b), much reduced in S. dispar 
and S. insignis (pl. LX), and particularly so in S. marginata. 
The Antennule. In all species, except the five without separate head and without 
frame, the antennule 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. pl. IL, fig. 1h and fig. 3a, pl. XI, fig. 1a). 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 sete; 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 antennule become 2-jointed 
by the fusion of the second and third joints (pl. XLII, fig. 1d). In Spher. decorata (pl. VIL, 
fig. 3e) the first and second joints are coalescent. In Spher. marginata the antennule 
(pl. XIII, fig. Gd) are constructed as in Homoeoscelis. In the other four species of Spheronella, 
which are devoid of separate head and of frame, the antennule 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 Antenne. These organs I have been unable to discover in the species of the 
genus Homoeoscelis, and in Spher. modesta, S. dispar, S. insignis, S. marginata, S. Munnopsidis 
and S. micerocephala, whereas they exist in the other species of Spheronella 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. pl. V, fig. 2d), 
without hairs and terminating in one shorter or longer seta. In a few species, e.g. Spher. 
decorata, the number of joints is reduced to two, in Spher. antillensis (pl. III, fig. 2c) they are 
