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.
* Since writing the above, I have been fortunate in procuring a very good specimen of Astacus fluviatilis, obtained soon after casting its shell, and also its newly cast-off covering, both of which are in excellent preservation. The following is a description of the appearances observed in each: - All the pieces of the exuvise were connected together by the old articulations, and accurately represented the external form of the complete animal, - the carapace, or dorsal shield of the cephalothorax, alone being detached, having been thrown off in one piece. The pedicles of the eyes and external corneae, as well as the antennae, remaind in situ, the corresponding parts having been drawn out from them as the finger from a glove; and no fissure of the shell or rupture of the ligaments connecting the joints was anywhere visible in these portions of the skeleton. The auditory tubercles, and the membrane stretched over the orifice of the ear, occupied the same position as in the living Cray-fish. The jaws, foot-jaws, and ambulatory feet retained their original connexions, with the exception of the right chela, which had been thrown off before the moult began; and the segments of the abdomen, false feet, and tail-fin exactly resembled those of the perfect creature.
Even the internal processes derived from the thoracic segments (apode-mata) rather seemed to have had the flesh most carefully picked out from among them, than to have been cast away from a living animal. But perhaps the most curious circumstance observable was, that attached to the base of each leg was the skin which had formerly covered the branchial tufts, and which, when floated in water, spread out into accurate representations of those exquisitely delicate organs. No fissure was perceptible in any of the articulations of the small claws; but in the chela each segment was split in the neighbourhood of the joints, and the articulating ligaments ruptured. The lining membrane of the stomach was found in the thorax, having the stomachal teeth connected with it: from its position, it would seem that the animal had dropped it into the place where it lay, before the extrication of its limbs was quite accomplished. The internal tendons were all attached to the moveable joint of each pair of forceps, both in the chela and in the two anterior pairs of smaller ambulatory legs.
(1005). The structure of the articulations which unite the different segments of the skeletons of the Articulata, and the general arrangement of their muscular system, have already been described; and in the class before us, these parts of their economy offer no peculiarities worthy of special notice.
(1006). Throughout all the Crustacean families the alimentary canal exhibits great simplicity of arrangement, and consists of a short but capacious oesophagus, a stomachal dilatation or cavity in which is contained a singular masticatory apparatus, and a straight and simple intestinal tube, which passes in a direct line from the stomach to the last segment of the abdomen, where it terminates.
(1007). The description of these parts as they exist in the Lobster, will give the reader a sufficiently correct idea of their general disposition and structure; nor are we acquainted with any class of animals in which so little variety in the conformation of this portion of the system is to be met with.
(1008). The oesophagus is covered at its origin by the several pairs of foot-jaws already alluded to, the most internal of which forms a decided cutting apparatus, resembling a pair of strong shears, while the rest are only instruments of prehension, or, perhaps, of sensation also.
On examining the animal which had extricated itself from the exuviae described above, the shell was found to be soft and flexible, but contained a sufficiency of calcareous matter to give it some firmness, especially in the claws. The tendons of the forceps were still perfectly membranous, presenting a very decided contrast when compared with the old ones affixed to the discarded shell. The stump of the lost chela had not as yet begun to sprout, and the extremity was covered by a soft black membrane. The jaws were quite hard and calcified, as likewise were the teeth contained in the stomach.
From the mouth, the oesophagus runs directly upwards to the stomach, which is a considerable viscus (fig. 201, a), a large portion of it being situated in that region of the cephalothorax which we should be tempted to consider as the head of the animal. The pyloric extremity of the stomach is strengthened with a curious framework of calcareous pieces imbedded in its walls, and so disposed as to support three large teeth placed near the orifice of the pylorus; and being moved by strong muscles, teeth so disposed no doubt form an efficient apparatus for bruising the food before it is admitted into the intestine.
(1009). The intestine itself (b b b) runs in a direct course to the tail, imbedded between the two great lateral muscular masses that move the abdominal segments, and terminates upon the ventral surface of the central lamella of the terminal fin, in a rounded orifice closed by a sphincter muscle.
(1010). The lrver (c c c), one half of which has been removed in the engraving, consists of two-large symmetrical masses, enclosing between them the pyloric portion of the stomach and a third part of the length of the intestine. When unravelled, the minute structure of the liver exhibits an immense assemblage of secerning caeca agglomerated into clusters, from each of which a duct emanates; and the continued union of the ducts so formed ultimately gives origin to the common hepatic canal (d), which pours the bile derived from that division of the liver to which it belongs into the intestine, at a very short distance from its commencement at the pylorus. A little below the insertion of the two bile-ducts, a solitary long and slender caecum enters the intestine; but the nature of the secretion furnished by this organ is unknown.