PACKARD.] MORPHOLOGY OF PHYLLOPODA. St 
to the new shell underneath, and thus the “lines of growth” correspond 
to the successive molts of the animal. Jn his First Book of Zoology, p. 
149, he remarks: “The concentric lines on the shell appear like lines of 
growth, and such they really are; but they are not made like the lines 
of growth on the mussel. When the creature molts the delicate skin 
covering the antenne and swimming legs is discarded. The molting 
process also takes place with the bivalve shell; but, instead of its being 
discarded, the molt is held or cemented to the new shell, which forms 
underneath. Molt after molt of the shell is thus retained, the increasing 
size of each molt showing as separate concentric lines of growth. If the 
shell is cut into and the cut edge is examined with a microscope, the 
successive molts will be seen resting one upon the other, like the leaves 
of a book.” This view would seem, at first sight, to be borne out by the 
relation of the marginal row of spinules, which are present in most 
species of Hstheria, as seen in our figures of the edge of the carapace of 
Estheria jones (Plate XXIV, fig. 2), where there are four marginal 
rows of round sockets, which must originally have borne spinules like 
the marginal ones. But our sections of the shells of Hstheria mexicana 
show that Morse’s view is not tenable, as the shell, if anything, is thicker 
at the edge than near the hinge, and there are no overlapping lines of 
growth.* An inspection of the broken shell of H. jonesii (Plate XXIV, 
fig. 2) shows that the ridges or so-called lines of growth are superficial, 
and, like the rows of beads and tubercles on the shells of the other 
species, together with the spines themselves, are merely external orna- 
mentation; for when the shell is broken and split, as in our fig. 2 of 
this plate, they are seen not to extend through the shell, there being 
irregular, not parallel, structural, lines in the substance of the shell. 
That the entire shell is molted with the integument or cuticula of the 
head and appendages is also shown by the fact that the carapace-valves 
of Limnetis show no such lines of growth, nor the carapace of Apus. So 
that, in respect to the casting of the carapace, the process in the Limna- 
niad& is not an exception to that in other crustacea where the cuticula 
of the entire body-wall is cast at once. Hence it would appear that the 
so-called “lines of growth” may be simply a superficial ornamentation, 
the ridges differing in different species. 
That the shell of Hstheria mexicana is cast at each molt is shown by a 
number of sections where the new chitinous shell is seen lying next to 
the hypodermis and the shell about to be cast is split off; also near the 
hinge, and especially over it, the shell is absorbed, so that the hinge 
margin is not.cast. The old shell was also in our sections divided into 
three layers. Claus also states that Hstheria mexicana casts its shell. 
It is probable that each species of Estheria has a row of spinules 
along the edge of the carapace-valves; we have found these spinules 
very long and slender in Hstheria belfragei and LH. mexicana, very short 
in H. jonesit and H. lindahli. We have not observed any in LE. califor- 
nica, nor has Lenz. 
*According to Joly and Klunzinger in Hstheria and Limnadia during the moulting as 
seen in repeated precise periods, only the delicate layer lining the inside of the shell 
is cast off with the skin of the telson, the hardened lamellated outer layer not only 
remains, but forms each time a new marginal zone. Since the inner layer is a direct 
continuation of the delicate body-skin, so is a periodical renovation of the same through 
a process of formation arising from the underlying matrix evidently found at each 
moulting of the telson; at the same time, however, this matrix, while it adds to the 
extent of the surface, also externally produces a new layer which, on the other hand, 
lies under that last formed and projects from the edge. In this way with the general 
growth of the shell, not only the lamelle overlying one another, but also the concen- 
tric lines, each one of which corresponds to a line of growth, find their simple expla- 
nation, and hence the view of Claus, who considered that the whole shell was cast at 
each moulting and was newly formed, cannot be the true one, 
