those which are most familiarly known, the Lobsters, Crabs, Shrimps, &c., being comprised under this head. For the most

part they are aquatic in their habits, and they are usually protected by strong resisting shells. There is always a complicated set of "gnathites," or appendages modified for masticatory purposes, surrounding the mouth. The ambulatory feet are made up of five pairs of legs (hence the name of the order); the first pairand often some other pairs behind this-being "chelate," or having their extremities developed into nippingclaws. The branchia are pyramidal, and are contained in cav ities at the side of the thorax. The carapace is large, covering the head and thorax, and the anterior part of the abdomen. The heart of the Decapoda is in the form of a more or less quadrate sac, furnished with three pairs of valvular openings. As regards the development of the Decapods enormous differences obtain, even amongst forms very closely allied to one another.

The Decapoda are divided into three tribes, termed respectively the Macrura, Anomura, and Brachyura, and characterised by the nature of the abdomen.

TRIBE A. MACRURA.-The "long-tailed" Decapods included in this tribe are distinguished by the possession of a welldeveloped abdomen, often longer than the cephalothorax, the posterior extremity of which forms a powerful natatory organ or caudal fin. As regards the development of the Macrura, most appear at first in the form of "Zoeæ;"* but there is

The young Decapod, in most cases, leaves the egg in a larval form so different to the adult that it was originally described as a distinct animal under the name of Zoea. In this stage (fig. 143) the thoracic segments with the five pairs of legs proper to the adult are either wanting or are quite rudimentary. The abdomen and tail are without appendages, and the latter is composed of a single piece. The foot-jaws are in the form of natatory forked feet, and the mandible has no palp. Lastly, there are

little metamorphosis in the common Lobster, and there is said to be none in the Cray-fish (Astacus fluviatilis). Fritz Müller,

Fig. 141.-Macrura. Common Lobster (Homarus vulgaris). 1. First pair of legs, constituting the great chele or nipping-claws; 2. Second pair of legs, also chelate; 3. Third pair of legs, also chelate; 4 and 5. Last two pairs of ambulatory legs, with simply pointed extremities; a Antennules; ga Great antennæ; ca Carapace.

again, has shown that the primitive form of one of the Shrimps no branchiæ, and respiration is carried on by the lateral parts of the carapace. The "Zoea " is separated from the "Nauplius" by having a segmented body, large paired eyes (sometimes with a median eye), and a carapace. The form proper to the adult is not attained until after several moults, constituting a genuine metamorphosis, though one which is effected by very gradual stages.

(Penæus) is that of a "Nauplius." This section comprises the Lobster, Cray-fish, Shrimp, Prawn, &c., of which the Lobster may be taken as the type.

In the Lobster (figs. 122, 123, and 141) the somites of the head and thorax are amalgamated into a single mass, the "cephalothorax," covered by a carapace or shield, which is developed from "the lateral or epimeral elements of the fourth cephalic ring, which meet along the back, and give way preparatory to the moult. The tergal elements of the thoracic rings are not developed in either Crabs or Lobsters; when these rings are exposed by lifting up the cephalothoracic shield, the epimeral parts alone are seen, converging obliquely towards one another, but not joined at their apices" (Owen).

The first segment of the head bears the compound eyes, which are supported upon long and movable eye-stalks or peduncles. Behind these come two pairs of jointed tactile organs, the larger called the "great antennæ," the smaller the "antennules." The mouth is situated on the under surface of the front of the head, and is provided from before backwards with an upper lip ("labrum"), two "mandibles," two pairs of "maxillæ," three pairs of "maxillipedes" or "foot-jaws,” and a bifid lower lip, or "metastoma." The five remaining segments of the thorax carry the five pairs of ambulatory legs, of which the first constitute the great claws, or "chelæ; the next two pairs are also chelate, though much smaller; and the last two pairs are terminated by simply pointed extremities. The segments of the abdomen carry each a pair of natatory limbs, or "swimmerets," the last pair being greatly expanded, and constituting, with the "telson," a powerful caudal fin. Most posteriorly of all is the post-anal plate, or "telson," which may be looked upon either as an azygous appendage, or as a terminal segment which has no lateral appendages.

The mouth leads by a short cesophagus 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 welldeveloped 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. 123, 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 laminæ, 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 maxillæ, 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 antennæ, 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 rudimentary 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 (Pagurida). In the common Hermit-crab (Pagurus Bernhardus) 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 (Porcellana), and the Tree-crabs (Birgus).

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. 142) 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