outer surface is delicately marked off into eighty or a hundred rings, of which from three to five are included in one of the deeper true segments corresponding to those of other annelids. From two to ten pairs of simple eyes are borne on the head, and owing to the fact that they are active swimmers, or move by caterpillar-like looping, locomotor spines are unnecessary and absent. In their internal organization, however, there are many features which indicate a close relationship with the Oligochates or fewbristle worms. The nervous, circulatory, and certain characteristics of the excretory systems are decidedly similar, but, on the other hand, there are some facts difficult to explain, which have led some zoologists to believe that the relationship of these animals can not at present be determined. 67. Haunts and habits.-The leeches usually dwell in among the plants in slowly running streams, but some occur in moist haunts on land, and a considerable number live in the sea. All are "bloodsuckers"-fierce carnivorous worms, whose bite is so insidiously made that the victim frequently is ignorant of their presence. Fishes, frogs, and turtles are the most frequently attacked, but cattle and other animals which come down to drink also become their prey. In some of the tropical countries the land-leeches are present in large numbers secreted among the leaves, and so severe are their attacks that various animals, even man, succumb to their united efforts. Adhering by their suckers, they puncture the skin, some using triple jaws, and fill themselves until they become greatly distended, when they usually drop off and digest the meal at leisure. In certain species the intestine is provided with lateral pouches (Fig. 40), which serve to store up the food until the time for digestion arrives. A full meal is sufficient with some species to last for two or three months, and the medicinal or horse-leech when gorged with food may consume a year in digesting it. 68. Egg-laying.-The eggs of some leeches are stored up in a cocoon like that of the earthworm, which is attached to submerged plants or placed under stones. When the young are able to lead independent lives they emerge with the form of the parent. A leaf-like form, Clepsine, sometimes found adhering to turtles, fastens the eggs to the under side of its body, and the young when hatched remain there for several days, adhering by their posterior suckers. CHAPTER VII ANIMALS OF UNCERTAIN RELATIONSHIPS 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. 69. 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 dispels 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. 41.-A wheel 66 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. 70. 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. 42) 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. 92), they were earlier associated in the same group; but an examination of their internal organization inclines many zoologists 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 dis- gested 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. 71. 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 |