378 GEOLOGICAL SURVEY OF THE TERRITORIES. 
The structure of the hypodermis is seen in fig. 7, Plate XX XIII. The 
sets of curved fibers (/) are arranged in upright bundles, with some 
transverse fibers, the ends of the former radiating at the surface and 
forming the stellated appearance so characteristic of the surface of the 
integument in these Phyllopods. The spaces between the bundles are 
not hollow, as represented by Grube; but in the specimens we examined 
besides numerous smaller cells there is a very large central cell (I. ¢.), 
which is perhaps actively concerned in secreting the shell. 
The shell-gland (Plate XXIV, fig. 9, shg) is seen to be a specialized 
portion of the cellular layer of the carapace; the cells glandular in their 
nature and secreting the material for the shell or cuticle, which is dis- 
tributed by the three primary ducts represented by the six openings seen 
in the drawing. The structure of the shell-gland in the Phyllopoda has 
been fully described by authors, particularly by Leydig, Claus, ete. 
While the carapace is well developed in the Limnadiade, with the Apo- 
dide it is very much smaller, covering only the cephalothoracic portions, 
leaving the abdomen exposed, so far as cephalothorax and abdomen 
may be said to exist in the Phyllopoda. The carapace is largest in the 
lower species and smaller in what we regard the higher species in the 
genera Lepidurus and Apus, respectively. In this family, therefore, 
its small size in adult life is a sign of superiority; as when it is large 
and covers most of the abdomen, it approaches nearly the larval con- 
dition of the species, and also the Limnadiad nature of the carapace, 
and in this respect, as well as in regard to the head, the Apodide are 
but one step removed from the Limnadiade. 
It also appears that the carapace is, as in Decapods, due to the hyper- 
trophy of the tergum of the mandibular segment, the adductor muscle 
being situated immediately over the mandibular adductor muscles. 
In the adult Branchipodide the carapace is entirely wanting. In 
adult life the shell-glands persist (Pl. XXIII, figs. 1, 2, gl.). Thus the 
Branchipodide are the extreme in the Phyllopod series, and stand at 
the head of the suborder, and hence as regards the carapace the devel- 
opment of the individual Branchiopod is in a degree an epitome of that 
of the suborder; and we have already seen that this succession or rel- 
ative standing of the three families of Phyllopods accords with the geo- 
logical succession of the genera Ustheria, Apus, and Branchipus. 
Morphology of the head.—The relative size and form of the head va- 
ries greatly in the three families of the Phyllopoda. In Limnetis the 
head is enormous in size and about equals in bulk the rest of the body ; 
this is due to the great development of the sternal side, but especially 
of the tergal portion in front of the eyes. In respect to the great bulk 
of the head the Limnadiade, and especially Limnetis, are connecting 
links between the Cladocera and Phyllopoda. Im the Cladocerous gen- 
era Daphnia, and especially Acroderus as figured by Leydig, the head 
and particularly the frontal region is greatly developed, though much 
less specialized than in the Limnadiada. For example, the front is 
produced into a large, broad, solid preocular subregion, forming the 
rostrum, which is acutely mucronate at the tip in the females, broad and 
truncate in the males; behind this is a broad, solid region in which 
the eyes are situated. The head, in fact, may be divided into a distinct, 
specialized preoral antenniferous and oculiferous, or sensory; and into 
a postoral region; the preoral region in Limnetis and Hstheria may be 
subdivided into two, namely, a preocular and an ocular subregion. In 
Limnetis the preoral region is separated from the rest of the head by a 
deep suture, and in Hstheria there is a deep dorsal incision, allowing a 
considerable play of this region upon the postoral region. In Limnadia 
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