and the next is furnished with appendages, which are essentially the same in structure, but are much larger, constituting the great claws. The next two segments of the thorax, and

ca

a

2

3

Fig. 123-Morphology of Lobster. 1. Lobster, with all the appendages, except the terminal swimmerets, removed, and the abdominal somites separated from one another: ca Carapace; Telson. 2. The third abdominal somite separated: Tergum; s Sternum; Pleuron; a Protopodite; b Exopodite; c Endopodite. 3. One of the last pair of foot-jaws or maxillipedes: c Epipodite; g Gill; the other letters as before.

the segment in front of these (by some looked upon as belonging to the head, by others as referable to the thorax), bear each a pair of modified limbs, which are termed "maxillipedes," or "foot-jaws." These are simply limbs with the ordinary

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structure of protopodite, exopodite, endopodite, and epipodite, but modified to serve as instruments of mastication, the hindmost pair being less altered than the two anterior pairs (fig. 123, 3). The next two somites carry appendages, which are in the form of jaws, and are termed respectively the first and second pairs of "maxillæ." Each consists of the parts aforementioned, but the epipodite of the first pair of maxillæ is rudimentary, whilst that of the second pair is large, and is shaped like a spoon. It is termed the "scaphognathite," and its function is to cause a current of water to traverse the gillchamber by constantly baling water out of it. The next segment carries the biting jaws, or mandibles; " each of which consists of a large protopodite, and a small endopodite, which is termed the "palp," whilst the exopodite is undeveloped. The aperture of the mouth is situated between the bases of the mandibles, bounded behind by a forked process, called the "labium," or "metastoma," and in front by a single plate, called the "labrum" (upper lip). The next segment bears the long antennæ, or feelers (fig. 122, a), each consisting of a short protopodite, and long, jointed, and segmented endopodite, with a very rudimentary exopodite. In front of the great antennæ are the next pair of appendages, termed the "antennules," or smaller antennæ (fig. 122, a), each composed of a protopodite, and a segmented endopodite and exopodite, which are nearly of equal size. Finally, attached to the first segment of the head are the eyes, each of which is borne upon an eye-stalk formed by the protopodite. The gill-chamber is formed by a great prolongation downwards of the pleura of the thoracic segments, and the gills are attached to the bases of the legs.

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 a follows (fig. 124):-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 minute terminations of the arteries open into a series of irregular venous sinuses, whence it is collected into a principal ventral sinus, and distributed to the branchia, 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

Fig. 124-Diagram of the circulation of the Lobster. The systemic arteries are shaded longitudinally, the veins are dotted, and the branchial vessels are black. Heart; a a Systemic arteries; b b Branchial vessels; c c Venous sinuses; g g Branchiæ; Pericardium.

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 branchiæ, 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.

Distinct respiratory and circulatory organs may be altogether wanting; but, as a rule, distinct branchia 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 (Haustellata).

Order 1. Ichthyophthira.

SUB-CLASS I. EPIZOA (Haustellata).-The members of this subclass of the Crustacea are in the adult state parasitic upon the bodies of fishes, and are usually deformed; but in the young condition they are locomotive, and are furnished with antennæ and eyes. The mouth is suctorial, and the limbs are terminated by suckers, hooks, or bristles. There are no differentiated respiratory organs, but respiration is performed by the surface of the body. The males are rudimentary, and are much smaller than the females, which are usually furnished with external ovisacs. The Episoa are closely allied to the Copepoda, and may, indeed, be regarded as parasitic Copepods, having the mouth modified so as to form a suctorial tube or beak, resulting from the elongation of the labrum and labium. Within this are two stylets or lancet-shaped mandibles, used in piercing. The feet are often deformed by age, or wanting, but are primitively natatory. Not only does their developmental history bear out this view, but cases are known (in

some Lernaa) in which the males do not undergo retrograde metamorphosis, but remain permanently in the condition of free Copepods.

This division includes the single order Ichthyophthira, the characters of which are therefore the same as those of the subclass, comprising various parasites upon fishes belonging to the genera Lernæa, Achtheres, Peniculus, &c.

ORDER I. ICHTHYOPHTHIRA.-Adult parasitic, deformed, often with rudimentary limbs; mouth suctorial; respiratory organs wanting; females with external ovisacs. Larva locomotive, and undergoing retrograde metamorphosis.

The members of this order are attached in the adult condition to the skin, eyes, or gills of fishes, and when mature possess an elongated body, having a more or less distinct head, and in the females usually a pair of long, cylindrical ovisacs, depending from the extremity of the abdomen. Some adhere by a suctorial mouth, or by cephalic processes (Cephaluna); others are attached by a suctorial disc, developed at the extremities of the last pair of thoracic limbs, which are united together (Brachiuna); whilst in others (Onchuna) attachment is effected by hooks at the free extremities of the first pair of thoracic limbs (Owen).

Fig. 125.-Ichthyophthira. a Free-swimming larva of Achtheres percarum in its first stage; Adult male of the same. Enlarged. (After Owen.)

The males are usually not attached, but adhere to the females, of which, from their much smaller size, they appear to be mere parasites. The chief anatomical peculiarities of the female are the following:-The head is provided usually with a pair of jointed antennæ, and the body is divided into a cephalothorax and abdomen. The alimentary canal consists of a mouth, gullet, and intestine, terminating posteriorly in a distinct anus. The nervous system consists of a double ventral cord. The embyro (fig. 125, a) is free-swimming, and is provided with visual organs and locomotive appendages. The two sexes are now alike, and the conversion of the active embryo,