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are developed in almost every segment, whence they are finally swept to the exterior through the tubes of the kidneys (Fig. 43, B).
The Nereis and its immediate relatives are all active forms, and by means of powerful jaws, which may be quickly extended from the lower part of the mouth cavity, they capture large numbers of small crustaceans, mollusks, and worms which happen in their path. Others more distantly related make their diet of seaweed, and
numaa many living on the sea bottom swallow great quantities of sand, from which they absorb the nutritious substances.
73. Sedentary forms. —Preyed upon by many enemies, a large number of species have been forced to abandon an active existence save in their early youth, and to construct many interesting devices for their protection. Numerous species, shortly after they commence to shift for themselves, build about their bodies tubes of lime (Fig. 45), from which they may emerge to gather food and into which they may dash in times of danger. As the worm grows the tube is correspondingly enlarged, and these tubes, in all stages of construction and variously coiled, may be found on almost every avail Fig. 44.-A common able spot at the seashore, and may often marine worm (PO
lyno brevisetosa), be seen on the shells of oysters in the
with extended pro. markets. :
boscis and over. In other species the tube is like thin
lapping plates cov.
ering the back. horn, and may be further strengthened or concealed by numerous pebbles, bits of carefully selected seaweeds, or highly tinted shells, which give them a very attractive appearance. Such species usually develop out of immediate contact with other forms, but a few live so closely associated together that their twisted tubes
form great stony masses, sometimes several feet in diameter. 7 74. Effects of an inactive life.—In many species such a sedentary life has resulted in the almost complete disappearance of the lateral appendages, which therefore no longer serve as organs of respiration, and this function has been shifted accordingly on to other structures. These new organs are situated principally on the exposed head,
• Fig. 45.-Sedentary tube-dwelling marine worms, upper left hand Sabella (one-half
natural size), the remainder Serpula (enlarged twice). From life.
and Fig. 39 shows the general appearance of some common species. The corners of the mouth have expanded into great plumes, sometimes wondrously colored like a full-blown flower, and these, bounteously supplied with blood-vessels, act as gills. When disturbed, the plumes are hastily withdrawn into the tube, and some of the so-called serpulids (Fig. 45, bottom of figure) close the entrance with a funnel-shaped stopper. While the plumes are primarily respiratory organs, they also act as delicate feelers, and may even bear a score or more of eyes; and in addition, being
covered with cilia, create the currents of water which bring minute organisms serving as food within reach of the mouth.
75. Development.—Unlike the earthworms, the Polychætes lay their eggs in the sea water, where they are left alone to develop as best they may. Both the male and female Nereis, as the egg-laying time approaches, undergo remarkable changes in their external appearance, resulting in the form shown in Fig. 42, A. They are now active swimmers, and thus are able to scatter the fertilized
ph eggs over wide and more or less favorable areas. The young also for a time are free-swimming, but finally end their migrations by settling to the sea bottom, where they gradually attain the adult condition.
As in some of the flatworms, reproduction may also occur asexually by the division of the animal into two or more parts, each of which subsequently becomes a complete individual. In other species growth of various parts may result in two complete worms at the time of separation; and from such forms we may trace a fairly complete series up to those in which the original parent breaks up Fig. 46. -A leech (Macrobdel
la). Right-hand figure ilinto twenty to thirty young.
lustrates alimentary canal. 76. The leeches. — At first sight ph, pharynx ; C, crop ; P,
lateral pouches ; s.i., in. the leeches (Fig. 46), or at least the smaller, more leaf-like forms, might be mistaken for flatworms, especially for some of the parasitic species. As in the latter, the mouth is surrounded by a sucker, and another is located at the hinder end of the body, but beyond this point the resemblance ceases. The outer surface is delicately marked off into eighty or a hun. dred 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 Oligochætes 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.
77. 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. 45), 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.
78. 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.