. Text book of zoology. Zoology. 196 Arthropoda. Class 1. Crustacea. but special vessels are wanting. A well-developed shell-gland is present. Some Daphnids differ from the usual type now described, in that the carapace is wanting, or is only feebly developed, that the body is elongate, and that the thoracic-limbs are aberrant in form. The Daphnids are small (at most a few m/m. long), transparent animals, which are mostly fresh-water, though a few are marine; they move through the water by jumps (Water-fleas). During the summer, usually only females are found, producing partheno- genetically l
. Text book of zoology. Zoology. 196 Arthropoda. Class 1. Crustacea. but special vessels are wanting. A well-developed shell-gland is present. Some Daphnids differ from the usual type now described, in that the carapace is wanting, or is only feebly developed, that the body is elongate, and that the thoracic-limbs are aberrant in form. The Daphnids are small (at most a few m/m. long), transparent animals, which are mostly fresh-water, though a few are marine; they move through the water by jumps (Water-fleas). During the summer, usually only females are found, producing partheno- genetically large, thin-shelled " summer eggs," which are " brooded" in the cavity between the trunk and the carapace; the young ones leave the brood-pouch almost in the condition of the parent; in autumn males appear also. The fertilised ova, "winter eggs" ("resting eggs"), which are thicker-shelled than the summer ones, usually pass the winter inapeculiarcase {Ephippium], formed by the thickened cuticle of the whole or part of the carapace, and thrown off by the female together with the eggs. The winter eggs develop in the spring; in some forms the young ones hatch out of the winter as nauplii. Order 3. Xiphosura. In the living Xiphosura, which comprise only a single genus, Limulus (the King- crab), the body is divisible into two unsegmented parts, the cephalo-thorax and the abdomen, which are movably articulated; each of these regions is composed of a number of fused segments. The cephalo-thorax is strongly arched, the sides are thin and continued down to form a shield-shaped structure, which covers in the ambulatory appendages. A carapace is wanting. Upon the dorsal surface there is a pair of large. Fig. 157. Limulus polypJiemusi $, from below : reduced. 1—6 ambulatory appendages, 0 opercvilum of the gill-bearing- limbs, the edges of which are seen one behind the Please note that these images are extracted from scanned page images that m
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
