Fig. 201.—The Mantis Shrimp (Squilla Mantis)
Two species of Mantis Shrimps, one of which is represented in fig. 201, have been found off the south and south-west coasts, but these are not likely to be seen on the shore, since they inhabit deep water. Allied to these, and sometimes included with the Stomapods, are the Opossum shrimps, so called because the females of some species carry their eggs in a kind of pouch, thus reminding us of the marsupial quadrupeds of the same name. They are of very slender build compared with the mantis shrimps, and differ from them in that the carapace completely covers the thorax; but though this is the case, the fusion of the thoracic segments is not complete, since the posterior ones have still a certain amount of freedom of movement. Some species of opossum shrimps are abundant in the rock pools of our coasts, particularly in the south-west, but their bodies being often so transparent as to be almost invisible, they are consequently easily overlooked. Their general appearance may be gathered from our illustration of Mysis chamæleon, which is probably the most common species inhabiting our coast.
The highest crustaceans—the Decapods—are divided into two sub-orders—the Macrura, or Great-tailed, including lobsters, shrimps, &c.; and the Brachyura (Short-tailed), containing the crabs; but the number of British species is so large that it is impossible to give, in our limited space, a detailed description of all the commonest even. All we can do is to note a few of the more interesting features of certain species, to introduce such illustrations as will enable the young naturalist to identify a number of the commoner ones, and to give the general characteristics of the main divisions so that the student may be able to classify his specimens intelligently.
Fig. 202.—The Opossum Shrimp (Mysis chamæleon)
In the Macrura, as with other divisions of the crustaceans, we meet with very interesting modifications of the appendages, adapted to quite a variety of uses; and if the reader is unacquainted with these adaptations of structure to habit he cannot do better than secure a lobster or crayfish for study. It will be observed that the body may be divided into two main portions—the cephalothorax, consisting of head and thorax combined, and the abdomen. The former is composed of fourteen segments, so thoroughly fused together that they are denoted only by the fourteen pairs of appendages to which they give attachment, while the calcified skin forms one continuous shield surrounding the whole. The abdomen, on the other hand, consists of six distinct segments, each of which is surrounded by its own ring of the hardened integument, and is connected with its neighbours by means of a portion of uncalcified skin that renders the whole very flexible. A groove in the front portion of the great shield (carapace) marks the division between the head and the thorax, the former composed of six, and the latter of eight united segments.
