Guide to the Crustacea, Arachnida, Onychophora and Myriopoda exhibited in the Dept. of Zoology, British Museum (Natural History).

  • British Museum (Natural History). Department of Zoology.
Date:
1910
Catalogue details

Licence: In copyright

Credit: Guide to the Crustacea, Arachnida, Onychophora and Myriopoda exhibited in the Dept. of Zoology, British Museum (Natural History). Source: Wellcome Collection.

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    Wall- cases Nos. 1-3. carapace, and the fore part of the body has eight distinct somites each bearing a pair of walking legs. In front of these eight somites, which form what is called the “ thorax,” is the “ head,” a part of the body which is never, in any Crustacean, distinctly segmented, but which, since it bears five pairs of appendages, must contain at least five somites. The part of the body covered by the carapace of the Lobster includes the head and the thorax and is known as the “ cephalothorax.” It is necessary to remark, however, that the regions of the body named head, thorax, and abdomen in the Crustacea are by no means exactly equivalent to those so named in the other Arthropoda, for instance in Insects, and still less to the parts bearing the same names among Vertebrate animals. This “ segmentation ” of the body, or division into somites, is not only shown by the external covering, but affects some of the internal organs as well. Leaving these aside for the present, how- ever, and considering only the exoskeleton, the structure of a typical somite will be best un- derstood by examining one of the separated abdominal somites of the Lobster (Fig. 2). This consists of a ring of shelly substance, connected with the rings in front and behind by areas of thin membrane which permit movement in a vertical plane. For convenience of description the upper or dorsal part of this ring is called the “tergum” (or “tergite”) and the under or ventral part the “ sternum ” (or “ sternite ”). To the sternum are attached the appendages (or swimmerets), while the tergum overhangs the base of the appendage on each side as a flap called the “ pleuron.” The terminal segment of the body or “ telson ” never bears typical limbs, and on this account and also because of its mode of development in the embryo, it is not regarded as a true somite. The carapace of the Lobster is not formed simply by the terga of several adjacent somites becoming soldered together. This is shown by a comparison with some of the lower shrimp-like Crustacea (Mysidacea, see Table-case No. 5), in which the carapace Fig. 2. One of the abdominal somites of the lobster, with its appendages, separated and viewed from in front. [Wall-case No. 1.]