« AnteriorContinuar »
the eye of the squids and cuttlefishes has shown them to be remarkably complex and in many respects to be constructed upon much the same plan as those of the vertebrates. As to the other senses not so much is known, but undoubtedly many species of cephalopods are possessed of a shrewdness and cunning not shared by any other invertebrates, save some of the insects and spiders, and are vastly more highly organized than their molluscan relatives.
102. General characters.—In the Arthropods, that is, the crabs, lobsters, shrimps, insects, spiders, and a vast host of related forms, the body is bilaterally symmetrical, and is composed of a number of segments arranged in a series, as in the earthworm and other annelids. A hornlike cuticle, sometimes called the shell, bounds the external surface—in early life thin and delicate, but later relatively thick, and often further strengthened by lime salts. Along the line between the segments this coat of mail remains thin and forms a flexible joint. Appendages also are borne on each segment, not comparatively short and fleshy outgrowths like the lateral appendages of many of the worms, but usually long and jointed (hence the name Arthropod, meaning jointed foot), and variously modified for many different uses.
103. Classification. The species belonging to this group outnumber the remainder of the animal kingdom. Their haunts also are most diverse. Some are adapted for lives in the sea and fresh water, others for widely different situations on land, and a great number are constructed for a life on the wing. A certain resemblance exists among them all, but the modifications which fit them for their different habitats are also profound, and have resulted in the division of the Arthropods into five classes. The first class (Crustacea) contains the crayfish, crabs, etc.; the second (Onychophora) includes the curious worm-like peripatus (Fig. 72); the third (Myriapoda, meaning myriad-footed) embraces the centipeds and “ thousand-legs”; the fourth (Insecta) contains the insects; and the fifth (Arachnida) includes the scorpions, spiders, and mites.
104. The Crustacea.--The number of species of crustaceans is estimated to be about ten thousand, and while the greater number of these are marine, many are found in fresh water and a few occur on land. in size they range from almost microscopic forms to the giant crabs and lobsters. They differ also in shape to a remarkable degree, but at the same time there is a decided resemblance throughout the group, except in those species which have become modified by a parasitic habit. The characteristic external skeleton is invariably present, and gives evidence of the deep internal segmentation of the body. In the simple Crustacea this is very apparent, but in the higher forms it is usually more or less obscured, owing to the fusion of some of the different segments, especially those of the head, as in the crayfish (Fig. 65). .
The class of the Crustacea is subdivided into two subclasses (Entomostraca and Malacostraca), the first containing the fairy-shrimps (Branchipus, Fig. 59) and their allies, the copepods (such as Fig. 60), the barnacles (Fig. 61), and a number of other species. In their organization all are comparatively simple, usually small, and the appendages show relatively little specialization. The other subclass contains the more highly developed and usually large-sized Crustacea, among which are the shrimps, crayfishes, lobsters, crabs, and a number of other forms.
105. Some simple Crustacea.—While the members of the first subclass are minute and inconspicuous, several species are often remarkably abundant in our small fresh-water pools. Among these is the beautifully colored fairy-shrimp (Branchipus, Fig. 59), with greatly elongated body and leaf-like appendages, whose relatively simple character leads the zoologist to think that they are among the simplest
Crustacea, and in several points resemble the ancestral form from which all the modern species have descended. Some nearly related forms are provided with a great fold of the body-wall, which may almost completely conceal the animal from above, or it may be formed like a bivalve clam-shell, within which the entire body may be withdrawn. This
latter character is also found in the water-fleas (Daphnia), very much smaller forms, and sometimes occurring in millions on the bottoms of our ponds and marshes. They are readily distinguished from the fairy-shrimp by the shortness of the body, the small number of appendages, and by their habit of using their antennæ as swimming organs, which gives to their locomotion a jerky, awkward character.
106. Cyclops and relatives.— Cyclops (Fig. 60), the representative of a number of lowly forms belonging to the order of Copepods, is one of the commonest fresh-water Crustacea. The forward segments of the spindle-shaped body are covered by a large shield or carapace, the feet are few in number, and, like its fabled namesake, it bears an eye in the center of the forehead. Nearly related species are also remarkably abundant at the surface of the sea, at times occur.
ring in such vast numbers that they impart a reddish tinge to the water over wide areas, and at night are largely re
sponsible for its phosphorescence. Many others are parasitic in their habits, and scarcely a salt-water fish exists but that at one time or an. other suffers from their attacks. On the other hand, many fresh- and salt-water fishes depend upon the free-swimming forms for food, and hence, from an economic point of view, they are highly important organisms.
107. Barnacles.— The parasitic habit and the lack of locomotion has also produced marvelous changes among the bar
nacles, so great that Fig. 60.-Cyclops. e. 8., eggs ; i, intestine; ov, originally they were reproductive organ.
placed among the mollusks; and as with the parasitic copepods, their true position was only known after their life-history had been determined. In the goose-barnacles * the body, attached by a fleshy stalk to foreign objects, is enclosed by a tough membrane, corresponding to the carapace of other Crustacea, in which are embedded five calcareous plates. This
* So called because of the belief, which existed for three hundred years prior to the present century, that when mature these animals give birth to geese.