378 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



The structure of the hypodermis is seen in fig. 7, Plate XXXIIT. 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. c), 

 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, jjarticularly by Leydig, Claus, etc. 



While the carapace is well developed in the Limnadiadce, with the Ajpo- 

 didm it is very much smaller, covering only the cephalothoracic portions, 

 lea^dng 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 sijecies in the 

 genera Lepidurus and Apus, respectively. In this family, therefore, 

 its small size in adult life is a sign of superiority 5 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 Apodidm are 

 but one step removed from the Limnadiadce. 



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 Branchipodid^e the carapace is entirelv wanting. In 

 adult life the shell -glands persist (PI. XXIII, figs. 1, 2,\jl.). Thus the 

 Branchipodidm 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 Estheria., Apns, and BrancM/pus. 



Morphology of the head. — The relative size and form of the head va- 

 ries greatly in the three families of the Phyllopoda. In Ijimnetis 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 Limnadiadce, and esi>ecially Limnetis, are connecting 

 links between the Cladocera and Phyllopoda. Ih the Cladocerous gen- 

 era Baphnia, and es]jecially Acroderus as figured by Leydig, the head 

 and particularly the frontal region is greatly developed, though much 

 less specialized than in the Limnadiadce. Por 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 Avhich 

 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 Estheria maj' 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 

 deei) suture, and in Estheria there is a deep dorsal incision, allowing a 

 considerable play of this region upon the postoral region. In Limnadia 



