Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/667

Rh CRUSTACEA 633 atrophy, and frequently their entire structure having undergone modification, iu consequence of the peculiar existences which they lead. But our advance towards a more correct knowledge of the class, as a whole, has been mainly derived from the accumulated store of embryological and developmental studies which commenced in 1823 by Vaughan Thompson have in these later years been so successfully prosecuted by Audouin and Milne-Edwards, Darwin, Spence Bate, Van Beneden, Glaus, Anton Dohrn, A. S. Packard, O. F. Miiller, Fritz Miiller, and very many other naturalists. Much, however, still remains to be accomplished. The CRUSTACEA belong to the sub-kingdom ANNULOSA, and to the division ARTHROPODA, in which are also included the INSECTA, the MYRIAPODA, and the ARACHNIDA ; they are known also as ARTICULATA, from the body being composed of a series of distinct rings or segments, each segment usually possessing a pair of jointed appendages or limbs articulated to it. They may be defined to be those Articulata which, whenever respiratory organs a/e specially developed, possess branchiae and not tracheae. By this definition they are at once separated from all insects and myriapods, which invariably possess tracheao. But it remains a difficulty, if it be not altogether impossible in the present state of science, to frame any definition which shall similarly include all CRUSTACEA and exclude all ARACHNIDA. In both classes, in fact, there are forms which possess no special respiratory organs; and if in these cases we resort to other characters, none which are of universal application have as yet been discovered. It may be said, however, that as a rule these exceptional Crustacea possess more than four pairs of locomotive appendages, have two pairs of antennary organs, and possess a simple alimentary canal ; while the Arachnida generally have not more than four pairs of locomotive appendages, possess at most one pair of antennary organs, and have their alimentary canal produced into caeca (Huxley). EXTERNAL STRUCTURE. The skin of the Crustacea is more or less completely hardened by a horny deposit Al Fia. 1. Diagram figure of Gammarus locusta, Fabr. (after Spence Bate and J. 0. Westwood). C = cephalon or head ; Th = thorax ; Ab = abdomen. (See Table of Appendages, next page.) called &quot;chitine,&quot; with or without the addition of lime, 1 thus forming a defensive covering to the softer tissues of the animal. This hardened envelope serves also as an external skeleton, giving rigidity and support to the 1 Chevreul gives the following analysis of the shell of the common crab : Animal matter 28 6 Phosphate of lime 6 Carbonate of lime 62 8 Phosphate of magnesia 1 Soda salts, &c 1-6 100-0 internal organs ; and to its inflections, projections, and rugosities the muscles and membranes of the body and appendages are attached. BODY-SEGMENTS. The crustacean exo-skeleton consists of a series of rings, usually a repetition of each other, and differing only in modification according to the necessity of the various portions of the animal. Each of these divisions is called a somite (Huxley). The normal number of somites, or segments, is twenty- one; 2 but instances occur among the extinct Trilobita and the recent Phyllopoda and Branchiopoda, in which a larger number of segments than twenty-one are to be met with. On the other hand, many recent and fossil forms offer examples of Crustacea in which one or more segments are never developed ; but this apparent absence is generally due to their coalescence, and we shall not unfrequently find indications of this if we bear in mind the theory of Oken, that each pair of appendages indicates a separate segment. A knowledge, too, of the earlier (larval) stages of some of these forms 3 has revealed the presence of tho normal number of free segments in the young individual, which in later life are permanently soldered together. Although the segments of tho Crustacea are greatly modified in the different orders, yet they can nevertheless be shown to be all composed of two lateral moieties resembling each other ; we can also distinguish two arcs, the one superior, the other inferior, as shown in tho annexed figure (fig. 2). The superior central pair united constitute the tergum (t, t), and the lateral are ^ called the epimera (ep, ep). The inferior arc is composed of the same number of pieces. The two median pieces unite to form the sternum (s, s), and the latero-inferiorpair are called the episternum (es, es). They are always united at the sternum; but there generally exists, between the inferior arc and the epimera situated above, a wide space destined for the articulation of the corresponding appendage (Milne- Edwards). Mr C. Spence Bate, in his &quot; Report on the British Edriophthalmia &quot; (Brit. Assoc. Repts., 1855), differs from Milne- Ed wards and other previous writers who had con sidered the series of scale-like plates at the sides of the body-segments of the Amphipoda as representing the &quot; epimeral pieces &quot; of each somite ; on the contrary, ho considers them to be the dilated coxal joint (or protopodite) of each limb. This view he adheres to in his chief work 4 (1863, p. 3) on the sessile-eyed Crustacea, and reiterates in his Report 5 (1875, p. 47), where he writes the epimera as sectional pieces in a theoretical construction of a somite cannot exist ; they are really portions of the integument of the appendages. That they are present in the Amphipoda attached to the coxal joint of seven pairs of the limbs is 2 In all the higher Crustacea the body is normally composed of twenty-one segments, but, of these, the last never bears true ap pendages, and is developed subsequently to the others from the dorsal surface of the body. Hence we are justified in regarding it, not as a somite or primitive typical segment of the body, but as a peculiar median appendage, to which the special name of &quot;telson&quot; (Spence Bate) may be applied. Thus the number of somites becomes reduced to twenty, each bearing its pair of appendages (Huxley, Medical Times and Gazette, 1857, p. 507). Professor Bell considers the extremely minute and movable points attached to the extremity of this segment in Palce;non serratus to be a pair of rudimentary appendages (Hist. Brit. Stalk-eyed Crustacea, p. xx., 1853). 8 As, for instance, the larval stages of Limulus polyphemus. J. 0. Westwood, 1863. 5 British Association Reports t Bristol, 1875, published 1876. VI. So FIG. 2. Ideal segment.
 * History of British Sessile-eyed Crustacea, 2 vols., Spence Bate and