A deep-water crab often seen along Long Island Sound is the spider crab, or sea spider as it is incorrectly called by fishermen. This animal, with its long spiderlike legs, is neither an active runner or swimmer; it is, however, protectively colored like the dark mud and stones over which it crawls. The resemblance to the bottom is further heightened by the rough body covering, which gives a hold for seaweeds and sometimes sessile animals, as barnacles, hydroids, or sea anemones, to fasten themselves. Giant spider crab from Japan. From photograph loaned by the American Museum of Natural History. A spider crab from the Sea of Japan is said to be the largest crustacean in the world, specimens measuring eighteen feet from tip to tip of the first pair of legs having been found. Symbiosis. Certain of the spider crabs, as well as some of the larger deepwater hermit crabs, have come to live in a relation of mutual helpfulness with hydroids, sponges, and sea anemones. These animals attach themselves to the shell of the crab and are carried around by it, thus receiving a constant change of position and a supply of food. What they do for the crab in return is not so evident, although one large Chinese hermit regularly plants a sea anemone on its big claw; when forced to retreat into its shell, the entrance is thus effectually blocked by the anemone. The living of animals in a mutually helpful relation is called symbiosis. Of this we have already had some examples in plants as well as among animals. (See Lichens, page 172.) Habitat. Most crustaceans are adapted to live in the water; a few forms, however, are found living on land. Such are the wood lice, the pill bugs, which have the habit of rolling up into a ball to escape attack of enemies, the beach fleas, and others. The cocoanut crab of the tropics climbs trees in search of food, returning to the water, at intervals, to moisten the gills. Classification. All the forms of crustaceans mentioned belong to that subclass of crustaceans called the Malacostraca, the characters of the group being a definite number of segments and appendages. All having five pairs of walking appendages are called decapods. How many of the above-mentioned forms are decapods? ENTOMOSTRACA. - Another subclass of crustaceans, in which the number of appendages is not fixed, is the group Entomostraca. They are mostly small animals, some species existing in countless numbers. Such are the fairy shrimps found appearing in early spring in fresh-water ponds, little translucent swimming forms from one half to three fourths of an inch in length. Another fresh-water form often seen in aquaria is the water flea (Daphnia). From the economic standpoint, probably the most important crustaceans that we shall study are the copepods. These tiny animals are barely visible to the naked eye. They are found in almost every part of the world, from the arctic seas to those of the tropics, and in fresh as well as salt water. They are so numerous that the sea in places is colored by their bodies. So prolific are they that it is estimated that one copepod may produce in a single year four billion five hundred million offspring. These animals form a large part of the food supply of many of our most important food fishes as well as the food of many other aquatic animals. They are, then, in an indirect way, of immense economic value. - One of the most Cyclops, a common copepod, enlarged about twenty times. A mass of eggs at the right. Later, DEGENERATE CRUSTACEANS. interesting forms to a zoologist is the goose barnacle. This crustacean is free-swimming during its early life. however, after passing through several changes in form during its development, the barnacle settles down on a rock or some floating object, fastens itself along the dorsal surface, and remains so fastened during the rest of its life. Food comes to it in a current of water, which is set in motion by the rhythmical beating of the appendages. Thus food particles are carried along the ventral side of the body to the mouth. Such animals are said to be degenerate. PARASITIC CRUSTACEANS. Other crustaceans have become even more helpless and have come to take their living from other animals. In some cases they are simply a bag for absorbing nourishment from the host on which they are fastened. Such is the Sacculina, a degenerate crustacean that lives HUNTER'S BIOL. - 15 Group of acorn barnacles (closed), with starfish moving over the mass. Each shell contains a fixed, degenerate crustacean. attached to the body of the crab. Others attach themselves to fishes and are known to fishermen as fish lice. Characters Common to all Crustaceans. — In spite of all the differences in structure found in the various crustaceans studied, there are a number of characters possessed by all crustaceans in common. They have a segmented body covered with an exoskeleton; the latter conThe body is bilaterally symThe appendages are jointed Crustaceans tains chitin and carbonate of lime. REFERENCE BOOKS FOR THE PUPIL Burnet, School Zoology, pages 67-73. American Book Company. pany. Hunter and Valentine, Laboratory Manual of Biology, pages 138-146. Henry Holt and Company. FOR THE TEACHER Herrick, The American Lobster. Report of U.S. Fish Commission, 1895. Parker and Haswell, Text-book of Zoology. The Macmillan Company. XIX. INSECTS Insects and Crustaceans Compared. Both crustaceans and insects belong to a great group of animals which agree in that they have jointed appendages and bodies, and that they possess an exoskeleton. This group is known as the Arthropoda. Insects differ structurally from crustaceans in having three regions in the body instead of two. The number of legs (three pairs) is definite in the insects; in the crustaceans the number is not always fixed (as in the Entomostraca), but is always more than three pairs. The exoskeleton, composed wholly of chitin in the insects, is usually strengthened with lime in the crustaceans. Both groups have compound eyes, but those of the crustacea are stalked and movable. The crustaceans receive sensations of touch by means of sensory hairs which protrude through the exoskeleton. The other sense organs do not differ greatly. The most marked differences are physiological. The crustaceans take in oxygen from the water by means of gills, while the insects are air breathers, using for this purpose air tubes called trachea. The young of both insects and crustaceans usually undergo several changes in form before the adult stage is reached. They are thus said to pass through a metamorphosis. Both insects and crustaceans, because of their exoskeleton, must molt in order to increase in bulk. The insects are divided into a number of large groups called orders. The insects found in each order possess certain characters in common. We shall now examine several representatives from some of the different orders of insects commonly met with. THE ORDER ORTHOPTERA The Locust or Grasshopper. The locust (grasshopper) is a type of the class Insecta, which is characterized by possessing a body made up of segments, having jointed appendages, three pairs of legs, and breathing through a system of air tubes called Notice that the body is covered with an exoskeleton. This is composed of chitin, a substance chemically akin to that of a cow's horn. The grasshopper makes its home in fields of grass. Some species live in vacant lots where there is considerable earth exposed. Do such grasshoppers ever have the color of their surroundings? How might this be of service to them? Examine the legs of a living grasshopper, with a view to finding out their position when at rest. Examine the hind legs. Do you find any adaptations present which fit the legs for jumping? Examine the hooks and pads on the last segment or tarsus. Look for other adaptations. Besides flying and hopping, the grasshopper also crawls. In a resting position, it clings by means of the hooks and pads on the foot or tarsus. Spread out the wings. Note their position. Note any differences between the two pairs. Which pair would be most useful in flight? Notice the delicate lacelike underwings, the supporting veins of which are composed of tubes that carry blood and air. Notice the abdomen carefully. The most anterior segment is incomplete, and bears an oval structure, the tympanum, or ear drum. Count the number of complete segments in the abdomen. The female grasshopper has the free end of the abdomen modified for the purpose of egg laying. Note the two-parted structures making up the ovipositor or egg layer. The male has a more rounded abdomen. Observation of the abdomen of a living grasshopper shows a frequent movement of the abdomen. Count the number of movements in a minute. This is the breathing of the grasshopper. Along the side of the abdomen in eight of the segments (in the red-legged grasshopper) are found tiny openings called spiracles. A large spiracle may easily be found in the middle segment of the thorax. These spiracles open into little tubes called tracheæ. The trachea carry air to all parts of the body. By the movements of the abdomen just noted, air is drawn into and forced out of the trachea. 1 For laboratory directions see Hunter and Valentine, Manual, page 101. |