Fig. 137. - Morphology of Lobster. 1. Lobster, with all the appendages, except the terminal swimmerets, removed, and the abdominal somites separated from one another : ca Carapace; t Telson. 2. The third abdominal somite separated: t Tergum ; s Sternum; p Pleura; a Protopodite; b Exopodite; c Endopodite. 3. One of the last pair of foot-jaws or maxillipedes: e Epipodite; g Gill; the other letters as before.
As regards the digestive system of the Crustacea, the alimentary canal is, with few exceptions, continued straight from the mouth to the aperture of the anus. There are no salivary glands, but a large and well-developed liver is usually present. A heart is generally, but not always, present. In most of the lower forms it is a long vasiform tube, very like the "dorsal vessel" of Insects. In the higher Crustaceans, the course of the circulation is as follows (fig. 138): The heart is a muscular sac, situated dorsally, beneath the carapace, and it gives origin to six arterial trunks, which convey the aerated blood to all parts of the body. The terminations of the arteries open into a series of irregular venous sinuses, whence the blood is collected into a principal ventral sinus, and distributed to the branchiae, where it undergoes aeration. From the gills the now aerated blood is carried by a series of branchial vessels to a large sac, which is badly termed the "pericardium," and which envelops and surrounds the heart. The arterial blood gains access to the cavity of the heart by means of six pairs of valvular fissures, which allow of the ingress of the blood, but prevent regurgitation. A portion of the venous blood, however, is not sent to the branchiae, but is returned directly, without aeration, to the pericardium; so that the heart finally distributes to the body a mixture of venous and arterial blood.
Fig. 138. - Diagram of the circulation of the Lobster. The systemic arteries are shaded longitudinally, the veins are dotted, and the branchial vessels are black. h Heart; a a Systemic arteries; b b Branchial vessels; c c Venous sinuses; g g Branchiae; p Pericardium.
Distinct respiratory and circulatory organs may be altogether wanting; but, as a rule, distinct branchiae are present. The exact form and structure of the gills differ in different cases, but their leading modifications will be alluded to in treating of the different orders.
Tabular View of the Divisions of the Crustacea.
Sub-class I. Epizoa.
Sub-class II. Entomostraca.
" 11. Merostomata.
Tribe a. Macrura. " b. Anomura. " c. Brachyura.