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- month and at least three pairs of jaw-like appendages behind the month, in being nearly always of aquatic habits, and in breathing by gills or by the general surface of the body. A Crustacean can usually be distinguished from any other Arthropod by the fact that its “ walking-legs ” do not correspond in number or arrangement with those found in the other groups Thus an Insect can usually be recognised at first sight by haying three pairs of legs, an Arachnid by having four pairs, and a Centipede or a Millipede by having a great number of legs, all nearly alike. The Crustacea, on the other hand, show a great variety in the arrangement of their walking or swimming legs, but they very seldom exhibit any special resemblance, in respect of these appendages, to the other large groups of Arthropods. THE LOBSTER AS A TYPE OF CRUSTACEA. The plan of structure common to the whole Class will be best 3 understood by beginning with the study of a typical form. For this purpose the common Lobster has been selected as being easily accessible, of convenient size, and not too specialised to admit of ready comparison with other Crustacea. The Crayfish, which is the type more usually described in text-books, differs only in minor details from the Lobster. Like the other Arthropoda, the Crustacea have the body and limbs encased by a firm covering which gives support to the soft internal organs and in particular affords points of attachment for the muscles by means of which the animal moves. In other words, this covering plays the part of a skeleton; but since, unlike the bony skeleton of Vertebrate animals, it is outside instead of inside the soft parts, it is distinguished as an “ exoskeleton.” In many Crustacea also, the exoskeleton is sufficiently strong to serve the purpose of defensive armour, and to enable the limbs to act as efficient and powerful weapons. Although the firm outer covering is really continuous over the whole of the surface of the body and limbs, it becomes thinned away in places to form joints permitting movement between the various parts. Thus, the body and limbs are divided into “ segments ” * * The word “ joint,” often applied to these divisions of the body and limbs, ought properly to be restricted to the hinge or connection between two segments.