Imagens das páginas
PDF
ePub
[ocr errors][ocr errors][merged small][merged small]

In this chapter we shall consider in a brief way a number of different groups of animals whose relationships are uncertain. Up to the present time the study of their habits, structure, and development has been of too fragmentary or unrelated a character to enable the majority of zoologists to agree upon their classification. Nevertheless, many of

them are highly interesting and attractive, often very common, and in some respects they hold important positions in the animal kingdom.

79. The rotifers or wheel-animalcules.The rotifers or wheel-animalcules are relatively small and beautiful organisms, rarely ever longer than a third of an inch, but at times so abundant that they may impart a reddish tinge to the water of the streams and ponds in which they live. At first sight they might be mistaken for one-celled animals, but the presence of a digestive tract and of reproductive elements soon dis

pels such a belief. Examined under the animalcule (Rotifer). microscope, the more common forms are

seen to possess an elongated body terminating at the forward end in two disk-like expansions beset along the edges with powerful cilia. These serve to drive the animal about, or, when it remains temporarily attached

FIG. 47.-A wheel

by the sticky secretion of the foot, to sweep the food-particles down into the mouth. Through the walls of the transparent body such substances are seen to pass into the stomach, where they are rapidly hammered or rasped into a pulp by the action of several teeth located there. In the absence of a circulatory system the absorbed food is conveyed by the fluid of the body-cavity, which also conveys the wastes to the delicate kidneys. Several other features of their organization are of much interest, especially to the zoologist, who believes that he gains from their simple structure some ideas of the ancestors of the modern worms, mollusks, and their allies. During the summer the rotifers lay two sizes of "summer eggs," which are remarkable for developing without fertilization. The large size give rise to females, the smaller to males, the latter appearing when the conditions commence to be unfavorable. The “winter eggs,” fertilized by the males and covered with a firm shell, are able for prolonged periods to withstand freezing, drought, or transportation by the wind. The adults also are able under the same adverse conditions to surround themselves with a firm protective membrane and to exist for at least a year. Once again in the presence of moisture the shell dissolves, and in a surprisingly short space of time they emerge, apparently none the worse for the prolonged period of quiescence.

80. Gephyrea.—There is a comparatively large group of worm-like organisms, over one hundred species in all, which at present hold a rather unsettled position in the animal kingdom. Some of the more common forms (Fig. 48) living in the cracks of rocks or buried in the sand, usually in shallow tide pools along the seashore, have a spindleshaped body terminated at one end by a circlet of tentacles which surround the mouth. On account of their external resemblance to many of the sea-cucumbers (Fig. 95), they were earlier associated in the same group; but an examination of their internal organization inclines many zoologists

[ocr errors]

to the belief that the ancestors of some of these animals were segmented worms whose present condition has arisen possibly in accordance with their sluggish habits. This view is strengthened by the fact that in a very few species

the larvæ are distinctly segmented, but lose this character in becoming adult. As before mentioned, the greater number of species live in bur

rows in the sand n.C.

or crevices in the rocks, from which they reach out and gather in large quantities of sand.

As these substances FIE. 48.- A gephyrean worm (Dendrostoma). Specimen pass down the in

on left opened to show k, kidney, m, muscle banas, testine the nutri. - and n.c., nervc-cord.

tive matters are digested and absorbed, while the indigestible matters are voided to the exterior. When large numbers are associated together they are doubtless important agents in modifying the character of the sea bottom, thus acting like the earthworms and their relatives.

81. The sea-mats (Polyzoa).—The sea-mats or Polyzoa constitute a very extensive group of animals common on the rocks and plants along the seashore, and frequently seen in similar situations in fresh-water streams. A few lead lives as solitary individuals, but in the greater number of species the original single animal branches many times, giving rise to extensive colonies. In some species these extend as low encrusting sheets over the objects on which they rest; while in others the branches extend into the surrounding medium and assume feathery shapes (Fig. 49), which often bear so close a resemblance to certain plants

[graphic]
[graphic]
[graphic]

Fig. 49.-Lamp-shells or Brachiopods (on left of figure), fossil and living, and (on

right) plant-like colonies of sea-mats.

that they are frequently preserved as such. What their exact position is in the animal scale it is somewhat difficult to say; but judging especially from their development, it appears probable that they are distant relatives of the segmented worms.

82. Lamp-shells or Brachiopods.—Occasionally one may find cast on the beach or entangled in the fishermen's lines or nets a curious bivalve animal similar to the form shown in Fig. 49. These are the Brachiopods, or lampshells. The remains of closely related forms are often abundant as fossils in the rocks (Fig. 49). Over a thousand species have been preserved in this way, and we know that in ages past they flourished in almost incredible numbers and were scattered widely over the earth. Unable to adapt themselves to changing conditions or unable to cope with . their enemies, they have gradually become extinct, until to-day scarcely more than one hundred species are known. These are often of local distribution, and many are comparatively rare.

For a long period the Brachiopods, owing to their peculiar shells, were classed together with the clams and other bivalve mollusks. The presence of a mantle also strengthened the belief; but closer examination during more recent years has shown that the shells are dorsal and ventral, and not arranged against the sides of the animal as in the clams. Another peculiar structure consists of two great spirally coiled “arms,” which are comparable in a general way to greatly expanded lips. The cilia on these create, in the water currents which sweep into the mouth, the small animals and plants that serve as food. The internal organization resembles in a broad way that of the animals considered in the previous section, and it now appears that both trace their ancestry back to the early segmented worms.

83. Band or nemertean worms.In a few cases band or nemertean worms have been discovered in damp soil or in fresh-water streams. These are commonly small and incon- . spicuous, and are pigmies when compared with their marine relatives, which sometimes reach a length of from fifty to eighty feet. Many of the marine species (Fig. 50) are often found on the seashore under rocks that have been exposed

« AnteriorContinuar »