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are collected into two definite masses (Fig. 33, B), which constitute a simple brain on which the eyes are situated and from which bundles of nerve fibers pass to all parts of the body, the two extending backward being especially noticeable. As in the squirrel, these are distributed to the muscles and other organs to regulate their activity, while those distributed to the skin, especially in the forward part of the body, convey stimuli produced by touch. The branches connecting with the eyes enable the animal to distinguish light from darkness, but are probably too simple to allow it to clearly distinguish objects of the outside. world. The sense of smell and possibly that of taste are also present, but are relatively feeble.

Some other characters of this class will be noted in the consideration of the two following classes.

57. Parasitic flatworms (trematodes)-parasitism.-Mention has already been made of the associations of two animals as "messmates" for mutual benefit, such as the Hydractinia growing on the surface of the shell inhabited by the hermit crab, to which it gives protection by means of its nettle-cells, while in turn being borne continually into regions abounding with food. More frequently, however, one animal derives benefit from another without making any compensation. For example, many species of flatworms live within the shells of certain snails and upon the bodies of sea-urchins and starfishes, where they gather in their food supply safe from the attacks of enemies. Such associations are probably without much if any inconvenience to the animal thus inhabited, and it also appears probable that the tenants are transients, using the mollusk or starfish only as a temporary home. But from this condition of affairs it is only a short step to the parasitic habit, where the association becomes permanent and the occupant is provided with various structures which prevent its separation from its host. This latter kind of union occurs throughout the group of trematodes; all are parasitic, and

their internal organization, so closely resembling that of the free-living forms as to need no further description, indicates that they are

ter.

descendants of the lat-
In the greater
number the body is
flat, and a few species
still retain their outer
coat of cilia; but since
these are no longer of
service as locomotor
organs they have gen-
erally disappeared, and
in their place numer-
ous adhesive organs,
such as spines, hooks,
and suckers (Fig. 35),
have arisen, which en-
able the animals
hold on with great te-
nacity. Thus attached
to its host, and using
it as a convenient and
comparatively safe
means of locomotion,
the parasite may still

to

m

0

FIG. 35. - A parasitic flatworm (Epidella). m mouth; o, opening of reproductive system; s, sucker and spines for attachment. The digestive system is stippled; nervous system black. Enlarged 8 times, from Nature.

continue to capture small animals for food or may derive its nourishment from the tissues of the host. In addition there are numbers of internal parasites, living almost exclusively in the bodies of vertebrate animals, scarcely a single one escaping their ravages.

58. Life history. In the external parasites the young hatch out and with comparative ease make their way to another host; but the young of an internal parasite, inhabiting the alimentary canal, have a very slight chance indeed of ever reaching a similar location in another host.

For this reason an almost incredible number of eggs is laid, and some extraordinary measures are employed in effecting the desired result. Probably the best-known example is that of the liver fluke inhabiting the bile-ducts in the sheep. Each worm lays several hundred thousand eggs, which make their way from the host, and if they chance to fall in pools of water or damp situations may proceed to develop, otherwise not. If the surroundings be favorable, the young, like little ciliated Infusoria, escape from their shells and restlessly swim or move about for a short time, and if during this time they come in contact with certain species of snails living in these situations they at once bore into their bodies. Here they produce other young somewhat resembling a tadpole, that now make their escape from the snail. In a short time each one crawls upon a blade of grass, and surrounds itself with a tough shell, where it may remain for several weeks. If the grass on which they rest be eaten by a sheep, they finally make their way to the bile-ducts and there become adult. The life cycle is now complete; the young form has found a new host; and the process shows how wonderfully animals are adapted to the conditions which surround them, and how closely they must conform to these conditions in order to exist.

59. The tapeworms (cestodes). The cestodes, or tapeworms, are also parasitic flatworms in which the effects of such a mode of life are strongly marked. They occur almost exclusively in the bodies of vertebrate hosts and exhibit a great variety of bodily forms, in some cases resembling rather closely the trematodes, but in others strikingly different. In the latter type the body is usually of great length (from a few centimeters to upwards of sixteen meters (50 feet)), and terminates in a "head" (Fig. 36) provided, in the different species, with a great variety of hooks and spines and numbers of suckers for its attachment to the body of the host. From the head the body extends backward in the gradually enlarging ribbon-like body, slender at

first and scarcely showing the segments which finally become so prominent a feature.

When carefully examined, a two-lobed brain is found in the "head," and from it nerves extend the entire length of the body, followed throughout their

course by the tubes of the excretory system; also each segment contains a perfect reproductive system, so that even if it be separated from the others it may continue to exist for a considerable length of time. Furthermore, the tapeworms are surrounded by the predigested fluids of their host; a special alimentary canal is therefore superfluous, and all traces of it have disappeared.

FIG. 36.-Tapeworm (Tania

solium). In upper lefthand corner of figure is the much enlarged head. -After LEUCKART.

V60. Development.-As the animal clings in this passive way to the body of its host the segments, loaded with eggs ready for development, separate one after another from the free end of the body, pass to the exterior, and slowly crawling about like independent organisms, lay great numbers of eggs, which may find an intermediate host as in the life cycle of the liver fluke, and so in time find their permanent resting-place. Fortunately in all these parasitic forms, though an inconceivably great number of eggs are laid, only a comparatively few reach maturity. Even these, however, may cause at times great destruction among the higher, and especially our domestic, animals, often doing damage amounting to many millions of dollars per year.

61. The tapeworm in relation to regeneration.-It has been known for more than one hundred and fifty years that some of the lower animals possess to a surprising degree the ability to regenerate parts of the body lost through injury. The Hydra, hydroids, and some of the jelly-fishes

may be cut into a number of pieces, each of which will develop into a complete individual; and this power of recovery from the injuries produced by enemies is of the greatest service in the perpetuation of the species. This ability is also present in certain flatworms, and some species are known which voluntarily separate the body into two portions, each of which becomes an adult. In other species a similar process results in the formation of a chain of six individuals, placed end to end, the chain finally breaking up into as many complete worms. It is possible that the tapeworm may also be looked upon as a great chain of united individuals produced by the division of a single original parent, which becomes adapted for attaching the others until they separate. These latter are capable only of a very sluggish movement, and, devoid of mouth and alimentary canal, are not able to digest their food, but their life work is to so lay their eggs that they may develop into other individuals, and for this they are well adapted.

NEMATODES (THREADWORMS)

62. General characters.-This class of worms is composed of an enormous number of different species, some parasitic, others free all or a portion of their lives, and in view of the fact that they inhabit the most diverse situations it is remarkable that they are so uniform in their structure. In all the body is slender, and the general features of its organization may be readily understood from an examination of the "vinegar eel" (Fig. 37, A). This small worm (not an eel), a millimeter or two in length, lives on the various forms. of mold that grow in fermenting fruit juices, especially after a little sugar or paste has been added. A tough cuticle surrounds the body, preserving its shape and at the same time protecting the delicate organs against the action of the acids in which it lives. Through this may be seen great bands of muscles extending the entire length of the body and producing the wriggling movements of swimming

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