PACKARD.] MOEPHOLOGY OF PHYLLOPODA. 377 



to the new shell nnderneath, and thus the '' lines of growth" correspond 

 to the successive molts of the animal. In 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 antennae 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 Estheria, as seen in our figures of the edge of the carapace of 

 Estheria jonesii (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 Estheria 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 E. jonesii (Plate XXIV,, 

 tig. 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- 

 uiadse 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 difleiing in diiferent species. 



That the shell of Estheria 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 oil'; 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.. Glaus also states that Estheria 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 Estheria helfragei and E. mexicana., very short 

 in E. jonesii and E. lindahli. We have not observed any in E. califor- 

 nica, nor has Lenz. 



*According to Joly and Klnnziriger in Estheria 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 througk 

 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 lamellse 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 Glaus, who considered that the whole shell was cast at 

 each moulting and was newly formed, cannot be the true one. 



