The mouth leads by a short oesophagus into a globose stomach, in the cardiac portion of which is a calcareous apparatus, for triturating the food, which is commonly called the "lady in the lobster." The intestine is continued backwards from the stomach without convolutions, and the anal aperture is situated just in front of the telson. There is also a well-developed liver, consisting of two lobes which open by separate ducts into the intestine.
The heart is situated dorsally, and consists of a single polygonal contractile sac, which opens by valvular apertures into a surrounding venous sinus, inappropriately called the "pericardium." The heart is filled with oxygenated blood derived from the gills, and propels the aerated blood through every part of the body. The gills (fig. 137, 3, g) are pyramidal bodies attached to the bases of the legs, and protected by the sides of the carapace. They consist each of a central stem supporting numerous laminae, and they are richly supplied with blood, but are not ciliated. The water which occupies the gill-chambers is renovated partly by the movements of the legs, and partly by the expanded epipodite of the second pair of maxillae, which constantly spoons out the water from the front of the branchial chamber, and thus causes an entry of fresh water by the posterior aperture of the cavity.
The nervous system is of the normal "homogangliate " type, consisting of a longitudinal series of ganglia of different sizes, united by commissural cords, and placed along the ventral surface of the body. The organs of sense consist of the two compound eyes, the two pairs of antennae, and two auditory sacs.
The sexes are invariably distinct, and the generative products are conveyed to the exterior by efferent ducts, which open at the base of one of the pairs of thoracic legs. The ovum is " meroblastic," a portion only of the vitellus undergoing segmentation. The neural side of the body - that is to say, the ventral surface - appears on the surface of the ovum, so that the embryo is built up from below, and the umbilicus is situated posteriorly.
Tribe B. Anomura. - The Decapods which belong to this tribe are distinguished by the condition of the abdomen, which is neither so well developed as in the Macrura, nor so nidimentary as in Crabs. Further, the abdomen does not terminate posteriorly in a caudal fin, as in the Lobster. The development in the Anomura appears invariably to take place through Zoea-forms.
The entire group of the Anomura must be regarded as an artificial assemblage, composed of modified forms of both the Macrura and the Brachyura.
The most familiar of the Anomura are the Hermit-crabs (Paguridae). In the common Hermit-crab (Pagurus Bernhar-dus) the abdomen is quite soft, and is merely enclosed in a membrane, so that the animal is compelled to protect itself by adopting the empty shell of some Mollusc, such as the common Whelk, which it changes at will when too small. The Hermit is provided with a terminal caudal sucker, and with two or three pairs of rudimentary feet developed upon the abdomen, by means of which he retains his position within his borrowed dwelling. The abdominal appendages, however, are mostly unsymmetrical. The carapace is not strong, but the claws are well developed, one being always larger than the other. Other forms of the Anomura are the Sponge-crabs (Dromia), the Crab-lobsters (Porcellanae), and the Tree-crabs (Birgus).
Fig. 159. - Brachyura. The Spiny Spider-crab (Maia squinado).
Tribe C. Brachyura. - The "short-tailed" Decapods, or Crabs, are distinguished from the two preceding tribes by the rudimentary condition of the abdomen, which is very short, and is tucked up beneath the cephalothorax, the latter being disproportionately large. The extremity of the abdomen is not provided with any appendage, and it is merely employed by the female to carry the ova. The Crabs (fig. 159) are mostly furnished with ambulatory limbs, and are rarely formed for swimming, most of them being littoral in their habits, and some even living inland.
In all the essential points of their anatomy the Crabs do not differ from the Lobster and the other Macrura; but they are decidedly higher in their organisation. This is especially seen in the disposition of the nervous system, the ventral ganglia in the Crab being concentrated into a single large ganglion, from which nervous filaments are sent to all parts of the body. In the Land-crabs (Gecar-cinus) respiration is by branchiae, but there is almost always an aperture behind the carapace for the admission of air. They are distributed over the warm countries of the Old and New Worlds, as well as Australia. They are essentially terrestrial in their habits, and migrate in large bodies to the sea, in order to lay their eggs. Besides the true Gecarcini, members of other very different families live more or less constantly on dry land, and have air admitted directly into the branchial chamber. Amongst these are the Calling crabs (Gelasimus) and the Sand-crabs (Ocypoda).
Fig. 160. - Zoea of the Spiny Spider-Crab (Maia squinado), enlarged.
Reproduction in the Crabs is the same as in the Macrura, but the larva is exceedingly unlike the adult, and approximates closely to the type of the Macrura, another proof that the Brachyura stand higher in the Crustacean scale. The larval Crab was originally described as a distinct animal, under the name of Zoea (fig. 160), presenting in this condition a long and well-developed abdomen. It is only after several successive moults that the young Crab assumes its characteristic Brachyur-ous form, and acquires by gradual changes the features which distinguish the adult. The Zoeae of the Crabs are usually distinguished by the possession of long spines developed from the carapace. When first liberated from the egg, the Zoea is enveloped in a larval skin or membrane, which is shed in a few hours. Among the Land-crabs, there is no metamorphosis in Gecarcinus; but in some of the Gecarcinidae the young are Zoeae.