. Text book of zoology. Zoology. 192 Arihropoda. in large blood reservoirs^ whence it is passed on to the gills. After receiving fresh supplies of oxygen^ it travels through special vessels to the pericardium, enters the heart through the ostia, and then recommences the circulation. Excretory organs. Two pairs of tubular organs, probably representing the segmental organs of the Annelids, are found in the Crustacea. The inner opening is wanting; they are usually of con- siderable size and much coiled. The foremost pair,the antennary- glands, open on the basal joints of the second antennae; the
. Text book of zoology. Zoology. 192 Arihropoda. in large blood reservoirs^ whence it is passed on to the gills. After receiving fresh supplies of oxygen^ it travels through special vessels to the pericardium, enters the heart through the ostia, and then recommences the circulation. Excretory organs. Two pairs of tubular organs, probably representing the segmental organs of the Annelids, are found in the Crustacea. The inner opening is wanting; they are usually of con- siderable size and much coiled. The foremost pair,the antennary- glands, open on the basal joints of the second antennae; the second, the shell-glands,* at the base of the second maxilla. Both pairs are seldom developed in the same animal; frequently one pair is present, and atrophies later, when the other is Often both are absent. The great majority of Crustacea are of separate sexes. The genital organs open ventrally, as a rule at a considerable distance from the posterior end, and usually by two distinct openings. The apertures of the oviducts are, in many forms, further forwards than those of the vasa deferentia. Not infrequently, parthenogenesis occurs (see Branchipus, Apus, the Daphnids). The development of the Crustacea is usually connected with a very distinct metamorphosis, for the young one, when it leaves the egg, is essentially different from the adult. The difference depends, amongst other things, upon the smaller number of segments and limbs possessed by the larva ; and further upon the different structure and even function of these limbs. A large number of Crustacea leave the egg in the so-called n a u p 1 i u s-state ; as small, compact creatures, furnished only with the first and second antennae and the mandibles. These appendages are all de-. Fig. 153. Naupliua of Fenasus. After Pr. Miiller. * TUs name is connected with the fact that the glands often {, in Apus) lie for the most part in the carapace. t Among the Entomostraca, the shell-gland is vmiversaUy pres
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