To conclude, the Limuli, a group of Crusta ceans of the most singular conformation, are at the bottom of the scale in this respect ; for in them (fig. 411) the anterior ambulatory extre mities themselves surround the mouth, and their basilar articulations perform the office of jaws.
The organs of which we have just made mention, are, according to the modifications they undergo, adapted in a more or less espe cial manner to seize, to b.old fast, and to comminute the alimentary matters upon which the animal lives. Moreovez the thoracic ex tremities in many species are themselves calcu lated to accomplish one or all of these offices with various degrees of success, according to their form, their extent, and the mode in which they are terminated. The most favourable. disposition to these ends is observed in the lobsters, crabs, &c.; in a word in a very great number both of the short and long-tailed De capods, in which the anterior thoracic extre mities terminate in pincers of greater or less strength, armed with teeth and sharp hooks which give them increased powers of pre hension. This form results mainly from the state of extreme development in which the pe nultimate articulation frequently occurs, and its assumption of the shape of a finger, by the prolongation of one of its inferior angles. Against the finger-like process thus produced, which is of great strength and quite immove able, the last articulation can be brought to bear with immense force, as it is put into mo tion by a muscular mass of great size, and in relation with the extraordinary size of the pe nultimate articulation (the claws, pincers, or eheliferous extremities).
The extremity occasionally terminates in two articulations presenting no kind of unusual development, but the last of which, termi nated by a sharp point and armed with teeth or serrw, returns upon the preceding one, so as to form a kind of hook or pincer, opening in the opposite direction, (the- sub-cheliform extremities of the Spina and Crevettinx). Lastly, these extremities frequently terminate in a simple acute angle of which the animal can make rio use save in locomotion.
In the Sucking Crustacea, which live parasi tically on other animals and feed by sucking their blood, the structlfre of the oral apparatus is extremely different.* Certain pieces which
must be considered as analogous to the labium and languette, are elongated, so as to form a trunk or cylindrical tube, of variable length, adapted for sucking, and in the interior of which are lodged the mandibles, now pro longed so much that they form two slender and pointed processes the extremities of which serve as a lancet. The appendages which in the masticating Crustacea constitute the jaws, here continue rudimentary, and the three pairs of limbs which in the Decapoda complete the oral apparatus, under the name of maxillary extremities, are here transformed into organs of prehension, of different forms, by means of which the parasite attaches itself to its victim.
In the whole of the Crustacea the intestinal canal presents two openings, the mouth and the anus, always separated from each other i3y the whole length of the body.
The mouth is the mere an terior and outward expansion of the cesophagus; it is fur nished with nothing that can properly be compared to a tongue ; the horny and la _ _,, .
mei tar organ vvnich writers have sometimes spoken of under this name is nothing more than the lower lip, which has already been de scribed.
The (Esophagus itself is short ;- it rises vertically and runs to terminate directly in the stomach. Its general structure, as well as that of the stomach and whole of the intestinal canal, bears a very close resemblance to what we observe among. the superior animals. They each Consist of two membranous layers separated by one of muscular fibres, always of greatest thickness in those points in which the most energetic contractions take place, and especially at the entrance into and passage out of the stomach.
The stomach is of a globular form, and of very- great capacity ; it fills a considerable extent of the cephalic cavity, and presents two portions very distinct from one another; the cardiac region, vertically surmounting the mouth and cesophagus, the axis of which is lost in its own ; and the pyloric region, situ ated behind the former, and forming a right angle with it.