(335). The Helminthozoa, embracing the vast class of parasitic worms, may be conveniently divided into two groups. First, those which live as parasites - the Entozoa, and secondly, those which are free and have an independent existence, as is the case with many of the Trematode Worms and the Turbellariae.

* Quoy et Gaimard, Voy. de 1'Astrolabe.

(336). The Entozoa, as the name implies, are nourished within the bodies of other animals, from the juices of which they derive their sustenance. It may naturally be supposed that, living under such circumstances - deprived of all power of locomotion, debarred from the in-fluences of light, and absolutely dependent upon the fluids wherein they are immersed for nutriment - the Entozoa have little occasion for that elaborate organization needful to animals living in immediate communication with external objects.

(337). We find, therefore, among these creatures, certain races whose structure is of the simplest character possible, in adaptation to the circumscribed powers of which they are capable. Yet, however apparently insignificant some of them may appear, they not unfrequently become seriously prejudicial to the animals wherein they are found, by the prodigious numbers in which they exist, or from their growth in those organs more especially essential to life; and not a few of them, from their dimensions alone, sometimes prove fatal, as may be supposed from a mere inspection of the annexed figure (fig. 64), representing an Entozoon developed in the abdominal cavity of a fish.

(338). There are probably no races of animals which are not infested with one or more species of these parasites, from the microscopic infusoria up to man himself; and sometimes several different forms are met with in the same species, to which they would appear to be peculiar; nay, in some cases the Entozoa would seem themselves to enclose other species parasitically dwelling in their own bodies. Neither is their existence confined to any particular parts; they are met with in the alimentary canal, in the liver, the kidneys, the brain, the arteries, the bronchial passages, the muscles, the cellular tissue, and, in fact, in almost all the organs of the body.

(339). The Cystiform Helminthozoa, generally known by the name of Hydatids, are the simplest in structure; and with these, therefore, we shall commence our inquiry into the economy of these creatures. The Coenurus cerebrdlis (fig. 65), one of the most common, occurs in the brain of sheep, and is the cause of a mortal disease but too well known to the farmer; it is likewise occasionally developed in other ruminating quadrupeds, and, by partially destroying the cerebral substance, soon proves fatal. This Entozoon, represented in the figure of ordinary size, consists of a delicate transparent bladder, the walls of which, during the life of the creature, are visibly capable of spontaneous contractions on the application of stimuli. To this bladder, or common body, are appended numerous heads, which arc individually furnished with an apparatus of hooks and suckers (fig. 65, 2, a, b), calculated to fix them to the surrounding tissues.

Ligida simplieissima in the abdominal cavity of a Minnow.

Fig. 64. Ligida simplieissima in the abdominal cavity of a Minnow.

1. Caenurus cerebralis (nat. size.) 2. One head magnified: a, oral circlet of hooks; b, suckers.

Fig. 65. 1. Caenurus cerebralis (nat. size.) 2. One head magnified: a, oral circlet of hooks; b, suckers.

(340). The Oysticerci, or common hydatids, agree in the main features of their structure with the Coenurus, but are provided with only one head or oral orifice (fig. 66,2.) These animals are found in almost all the viscera of the body, and not unfrequently, especially in pigs, exist in great numbers, not only in the liver, which is their most usual seat, but in the cellular texture of the muscles, and even in the eyes themselves. The human frame is not free from their intrusion; and when they abound, serious consequences frequently result from theirpresence.

(341). Cestoidea

The Teenier, or tape-worms, arc among the most interesting of the Sterelmintha, whether we consider the great size to which they sometimes attain, or their singular construction. Several species of these worms infest the human body, and many other forms of them are met with in a variety of animals. They are usually found in the intestinal passages, where, being amply provided with nutritious aliment, they frequently grow to enormous dimensions, being not unusually twenty or thirty feet in length; and some have been met with much longer: it is therefore manifest how prejudicial their presence must prove to the health of the animals in which they reside; and we are little surprised at the emaciation and weakness to which they generally give rise.

1. Cysticercus iemiicollis (nat. size.) 2. Head magnified: a, circlet of hooks; b, suckers.

Fig. 66. 1. Cysticercus iemiicollis (nat. size.) 2. Head magnified: a, circlet of hooks; b, suckers.

(342). The Taenia solium, the species most usually met with in the human subject, at least in our own country, is that selected for special description. The body of this creature consists of a great number of segments united together in a linear series (fig. 67): the segments which immediately succeed to the head (a) are very small, and so fragile that it is rarely that this part of the animal is obtained in a perfect state; they gradually, however, increase in size towards the middle of the body (d.) The first joint of the Taenia, generally called the head, differs materially in structure from all the rest. This segment in the Taenia solium, when highly magnified, is found to be somewhat of a square shape; in the centre is seen a pore that has been considered to be the mouth, surrounded with a circle of minute spines, so disposed as to secure its retention in a position favourable for imbibing the chyle wherein it is immersed. Around this apparatus are placed four suckers, which are no doubt additional provisions for the firm attachment of the head of the worm. In other Taeniae the structure of the first segment is variously modified: thus, in Taenia lata the central pore has no spines in its vicinity; in Bothriocephalus there are only two longitudinal sucking disks; in Floriceps these are replaced by four proboscidiform prolongations, covered with sharp recurved spines, which, being plunged into the coats of the intestine, form effectual and formidable anchors; yet the intention of all these modifications is the same, namely, to retain the head in a position adapted to ensure an adequate supply of nutritious juices.