This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(981). Insects and Arachnidans are air-breathing animals; and even in such species of these two extensive classes as inhabit fresh water, respiration is strictly aerial. No insects or spiders are marine; and consequently the waters of the ocean would be utterly untenanted by corresponding forms of Articulata, were there not a class of beings belonging to this great division of the animal world so organized as to be capable of respiring a watery medium, and thus adapted to a residence in the recesses of the deep. Examined on a large scale, the Crustaceans, upon the consideration of which we are now entering, are marine creatures: many species, it is true, are found abundantly in the lakes and ponds around us; but these form rather exceptions to the general rule; and we may fairly regard this extensive group of beings as the aquatic representatives of the Insects and Spiders, with which they form a collateral series.
* Duges, Ann. des Sci. Nat 2 ser. torn. i. p. 165.
(982). The tegumentary system of the Crustacea corresponds in its essential structure with that of insects, and consists of a vascular dermis, a coloured pigment, and a cuticular secreted layer which forms the external shell or skeleton: the latter, or epidermic covering, however, differs materially in texture from that of other Articulata, inasmuch as it contains calcareous matter in considerable abundance, and thus acquires in the larger species great density and hardness.
(983). As regards the mechanical arrangement of the skeleton, we shall find the same general laws in operation as we have observed throughout all the Annulose orders - a continual centralization and progressive coalescence of the different rings or elements composing the external integument, and a strict correspondence between the degree to which this consolidation is carried and the state of the nervous system within.
(984). In the lowest forms of the Crustacea we have, in fact, a repetition of the condition of the skeleton met with in the Myriapoda, or in the larva state of many insects, - the whole body being composed of a series of similar segments, to which are appended external articulated members of the simplest construction (fig. 195).
(985). The number of rings or segments composing the body varies in different species; but such variation would seem, from the interesting researches of Milne-Edwards and Audouin concerning the real organization of articulated tegumentary skeletons, to be rather apparent than real, inasmuch as the discoveries of these distinguished naturalists go far to prove that, whatever the state of consolidation in which the integument is found, the same number of elements or rings may be proved to have originally existed before, by their union, they became no longer distinguishable as separate segments.
(986). The normal number of these elements Milne-Edwards considers to be twenty-one, seven of which enter into the composition of the head, seven belong to the thorax, and as many appertain to the abdominal region of the body.
(987). To illustrate this important doctrine let us select a few examples, in order to show the manner in which the progressive coalescence of the segments is effected.
(988). In Talitrus (fig. 196) the cephalic elements are completely united, their existence being only indicated by the several pairs of appendages - one pair, of course, belonging to each ring. The first ring of the cephalic region, in this instance, has no external articulated member; but in higher orders the eyes are supported upon long peduncles connected with this element of the skeleton, that may be regarded as the representatives of those limbs which take different names in different regions of the body. The second and third rings support jointed organs, here called antennce; while the several pairs of jaws appertaining to the mouth indicate the existence of so many elements united together in the composition of the head.
(989). The seven segments of the thorax are still distinct, and each supports a pair of jointed organs, which, being used in locomotion, are called legs; the abdominal elements, likewise, are equally free, and have natatory extremities developed from the five posterior rings.
(990). In the Lobster (Astacus marinus) we find not only the cephalic segments anchylosed together, but those of the thorax also; and although the lines of demarcation between them are still recognizable upon the ventral aspect of the body, superiorly the entire thorax and head are consolidated into one great shield (cephalothorax), the abdominal segments only remaining distinct and moveable.
(991). In the Crabs the centralization of the external skeleton is carried to still greater lengths, so as to enable this tribe of Crustaceans to become more or less capable of leaving their native element and walking upon the shores of the sea, or even, in some instances, of leading a terrestrial existence, as in the case of the Land-Crab of the "West India Islands. The abdominal segments, however, still remain free, though proportionately of very small dimensions; and, being no longer useful in swimming, the abdomen is folded beneath the enormously-developed thoracic portion of the body.
(992). In the King-Crab (Limulus Polyphemus, fig. 197) even the divisions of the abdomen are obliterated, the whole body being covered by two enormous shields, and the tail prolonged into a formidable serrated spine, of such density and sharpness that in the hands of savages it becomes a dreadful weapon, and is used to point their spears either for the chase or war.
Fig. 196. Talitrus.
(993). The reader will at once perceive the strict parallelism that may be traced between the changes which occur during the metamorphoses of Insects, and those observable as we thus advance from the lowest to the most highly organized Crustacean genera; and even the steps whereby we pass from the Annelidan to the Myriapod, and from thence to the Insect, the Scorpion, and the Spider, seem to be repeated, as we thus review the progressive development of the class before us.