On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

Anomura.

The Anomura is a family of Decapods intermediate between the long and short-tailed Crustacea. There are nine or ten genera and many species, chiefly distinguished by the development of the head and thorax, and the softness of a non-locomotive tail: of these the Pagurus, or Hermit crab, is assumed as the type or representative.

The carapace is long and convex, scarcely extending over the basal joints of the feet. The claw feet are short, with a very broad hand and sharp pincers; but the Hermit crab and some of its congeners are irregularly formed; for the last pair of walking feet, instead of being attached to the thorax, like the others, are fixed to the first part of the tail, are generally folded over the back, and are employed to sweep foreign matter out of the gills. The mouth and its masticating organs are similar to those in the crab, except the exterior pair of foot-jaws, which are longer and move like feet. But that which distinguishes the Pagurus and its fellows from every other Decapod is the softness of its unsymmetrical tail, all the appendages of which are abortive, and the extremity, instead of ending in a swimming fin, terminates in a pair of grasping organs. In order to protect this soft-skinned tail, the Hermit crab folds it up and thrusts it into some old empty shell, clasps the column of the shell with its grasping organs, draws in the rest of its body, and covers it with the broad hands folded in such a manner as to close the mouth of the shell, and to defend itself if attacked. It holds so fast that it cannot be drawn out; but, when in search of food, it stretches out its mailed head and legs, and walks off with its house on its back. However, it sometimes comes out of its shell to feed, and, like some other crustaceans, it holds its prey with one claw, and tears it to pieces with the other. They are very pugnacious, and come out of their shells to die. The larvæ of the Paguridæ undergo transformation, and they moult when full grown.

Stomapoda.

The Stomapods are all swimmers; they have long bodies with a carapace; but it is so varied in form and size, that no general description of it can be given. They have external, instead of internal, organs of respiration; gills in the form of tufts are in some cases attached to a few of the foot-jaws, but they are much more frequently fixed to the basal joints of their swimming feet, so that the blood in their capillary veins is aërated through their thin skin as they float in the water. In the Squilla mantis, or S. Desmarestii, members of a genus of this family, the gills, which are fixed to the basal joint of their last pair of feet, consist of a long conical tube, on each side of which there are numerous parallel tubes, like the pipes of an organ, and each of these has a row of many long cylindrical filaments that drag in the water. The mouth and its appendages are similar to those of the common Decapods, with the exception of the anterior jaw-feet, which are of a singular and formidable structure. They are bent outwards, and their basal joint is exceedingly large, broad, and compressed; the next joint is less, with a groove in its side; the third joint is a blade like a scythe, whose cutting edge is furnished with long pointed teeth. The Squillæ are carnivorous, and, if any unfortunate animal comes within their grasp, they bend back the toothed edge of the first joint into the groove of the second joint like a clasp-knife, and cut it in two. These prehensile foot-jaws, or ‘pattes ravisseurs,’ are like the fore-feet of the praying Mantis, and like them weapons of defence.

The genus Mysis, or Opossum Shrimps, have a long straight carapace, which covers most of the thorax, and folds down on each side so as to conceal the base of the feet: in front it is narrow, and ends in a flattened beak; at the posterior end it is deeply scooped out. The two last rings of the thorax are more or less exposed; the tail is long, almost cylindrical, tapering to the end, and terminating in a swimming fin composed of five plates spread like a fan. Both pairs of antennæ have jointed stems ending, the outer in one, the inner in two very long many-jointed filaments. On the top of the basal joint of the outer pair there is a very long lamellar appendage, ciliated on the side next the joint. Between the second and third joints of the exterior antennæ, Mr. Spence Bate found the organ of taste: the aperture is simply covered by a membrane, as in the lobster. The ears are in the last appendage of the tail.

The Mysis has two pairs of jaw-feet differing little from feet; five pairs of thoracic feet, all thin and divided into two branches, which increase in length as they are nearer the tail, and are all provided with a ciliated appendage to adapt them for swimming. In the female, broad horny plates, attached to the two last pairs of legs, are bent under the body so as to form a kind of pouch, destined to lodge the eggs and the young during the first period of their lives, whence their name, ‘Opossum Shrimps’: the young are crowded in this pouch, and acquire their adult form before they come into the water. The circulation of the white blood of the Mysis was discovered by Mr. Thompson: the pulsations of the heart are so rapid that they resemble vibrations. There are many species of these small shrimps.

Fig. 149. Lucifer, a stomapod crustacean.

The genus Lucifer is one of the most singular of the crustaceans from its almost linear form ([fig. 149]), the excessive length of the anterior part of the head, the extreme shortness of the thorax, the smallness of the carapace or shell, and the great development of the tail, which is more than three times as long as the thorax. The thin eye-stalks, which are of exaggerated length, extend at right angles from the top of the long cylindrical part of the head, and terminate in large, staring, dark-coloured eyeballs covered with a multitude of facettes. The two pairs of antennæ are placed between and below the eye-stalks. The undermost pair, which are the shortest, have a little lamellar appendage at their base: in some Lucifers, when viewed in front, it looks like a cross. The salient mouth is placed at the base of the long organ that carries the eye-stems. It has strong toothed mandibles, two pairs of jaws with plates attached to each jaw, and three pairs of foot-jaws. The tail is very narrow, consisting as usual of seven rings movable on one another; but they are quite abnormal, for each of the rings is at least as long as the thorax; the last has five plates spreading like a fan. All the bristly feet, which seem to hang loosely down from the animal, are fitted for swimming; those of the tail have long ciliated plates in their basal joints. These creatures are small, and inhabitants of warm seas.