The animals of this Class are parasitic, and are found only in the bodies of other animals. The Entozoa, or intestinal worms, are characterised in general by their want of an external or internal skeleton, by their naked, smooth, and elastic in tegument, by their very languid irritability, by their lengthened vermiform body, and by their very simple internal structure.
The external surface of the body is generally covered with a tough elastic and almost transparent homogenous integument. In some, as the the body is divided by numerous transverse depres sions, or articulations, like the surface of the anne lides. Muscular fibres have been observed in some of the species, forming two layers within the integ ument, the outer composed of transverse circular fibres, and the inner formed of longitudinal inter rupted fibres extending along the body. Nerves have also been observed in a few species, forming two simple lines, the one running longitudinally along the abdomen, the other along the back, and sometimes they appear to extend round the (eso phagus. The intestinal canal extends in a straight line along the body, without creca or convolutions, and has generally an anus distinct from the mouth. The mouth in the highest order, Entomoida, pre sents a proboscis adapted for sucking, with rudi mentary maxill?, and even jointed antennae. Eyes, and a system of circulation are said to exist in some of the remarkable epizoa belonging to this order. Distinct organs of respiration have not been observ ed, and it is very probable that their soft texture is originated by the fluids of the animals in which they live. Certain vessels passing longitudinally along the body are considered by some as trachea' for aquatic respiration, by others as vessels for circu lation, and by others as cceca from the intestinal canal. In the greater number of these animals the mouth is furnished with numerous sharp, hard, re curved teeth, by which they attach themselves to, and pierce through the bodies of animals. The most remarkable feature of their organization is the existence of separate sexes in beings of so sim ple a structure. A long convoluted spermatic ves sel, a seminal receptacle, and a projecting stiliform tubular body, have been observed in the male of some species, and in the female a kind of vulva com municating with an interior uterus, and two long slender white convoluted ovaria. In the Entomoi da the two ovaria generally hang externally from the posterior extremity of the body. Not only have the sexual organs been minutely described by several observers, but the animals have been seen to copulate.
The consideration of this class of animals is in teresting from the great variety of forms and struc tures they present, from the equivocal nature of their origin, and from the ravages they commit in the bodies of man and the inferior animals. Some naturalists, as Blumenbach, Lamarck, Mekel, &c. suppose them to originate spontaneously from the mere elements of the bodies in which they are found. Others suppose them to originate from animals taken in with the food or drink, and changed in form by the change of the medium in which they live. Others suppose them to originate from germs contained in the ovum, and thus transmitted by generation. They are found in animals, attached to the surface of the body, the lips, the fauces, the cornea of the eye, the interior of the stomach and intestinal canal, the bronchia, or the ducts leading from the intestine. They are found imbedded in the cellular texture of the extremities, in the mus cular substance of the body, in the parenchyma of all the internal organs, and even in the brain itself. These singular animals have engaged the attention of several eminent naturalists, and of many of the older medical writers. Redi has figured and des cribed several intestinal worms occurring in the lowest classes of animals, (an. 1671). Daniel Clerc published at Geneva in 1715 an elaborate treatise on these animals, illustrated with numerous magnified engravings (Hist. Nat. et Med Latorum Lumbri corum). LinnRus described only eleven species of intestinal worms in the last edition of his Systema Natures. Pallas has described the characters of
several species of the genus Tcenia. One of the most valuable works, however, which has yet ap peared on this tribe of animals, is the elaborate natural history of intestinal worms published at Blankenburg in 1782 by J. A. E. Goeze, honorary member of the natural history society of Berlin, (Versuch einer naturgeschichte der eingeweidewiir mer thierisher korper). This laborious and original work describing the dissected specimens of these animals in the author's extensive cabinet, is illustra ted with 44 beautiful quarto engravings, containing several hundred magnified objects. The numerous and accurate figures and descriptions of Goeze have formed the chief basis of all succeeding classifica tions of these animals. At the same time with the work of Goeze, a treatise on intestinal worms, and chiefly on the txnix of the human body, was pub lished by P. C. F. Werner at Leipsic, in one vol ume 8vo. with plates, and in the same year, 1782, M. E. Bloch published a medical treatise on these animals in 4to with 10 plates, at Berlin. In 1786 the Bibliotheca Helminthologica was published by Modeer at Erlangen, and in the same year A. I. Retzius printed his public lectures on intestinal worms chiefly from the human body, at Holm. Cuvier in his Tableau Elementaire, 1798, described a few of the genera, and placed them after the anne lides at the bottom of his great division of animals termed Insects and Worms. In his Regne Animal, 1817 and 1830, he has placed them below the Ech inedermata in the great division of Zoophytes, and divided them into two sections. viz. Cavitaires, (Nematoidea of Rudolphi), and Parenchimateux (comprehending the four last orders of Rudolphi). Bruguii.re, in the article Vers of the Encyclopedie Methodique, has given an extensive series of accu rate engravings illustrative of this class of animals, and a collection of folio lithographic plates, contain ing copies of all the more recent figures of Entozoa, has been just published in Germany. I. G. H. Zed er published at Bamberg in 1803 an introduction to the natural history of intestinal worms, in 8vo. with plates, in which work 391 species are enumerated. In the natural history of intestinal worms published at Amsterdam by Rudolphi in 1808-1810, in 2 vols. 8vo., 603 species are enumerated ; and in his Sy nopsis Entozoorum published at Berlin in 1819, he has enumerated more then eleven hundred species. The classification of Entozoa proposed by Rudolphi in this Synopsis, which is nearly the same as Zeder's, is that most generally adopted by naturalists, and it has been recently illustrated by a series of 18 folio engraved and coloured plates, executed under the direction of Dr. Bremser of Vienna. The French translation of the work of Bremser on the human entozoa has been enriched with numerous interesting observations by Al. Blainville. An interesting me moir on the anatomy of two species of intestinal worms was recently presented to the French Insti tute by Professor J. Cloquet, and has been publish ed in a separate form, illustrated with numerous highly magnified views of the internal organs. Blainville, Deslongchamps, and several other writers have likewise illustrated the structure of these ani mals. The classifications of Entozoa adopted by La. mark, Goldfuss, Schweigger, Latreille, and other modern systematic authors, are founded on the sys tem of Rudolphi, which we shall therefore lay before our readers in this article, with the addition of the Order Entomoida, composed of those more compli cated species which reside generally on or near the surface of aquatic animals. M. Latreille has given a very distinct and interesting view of the classifi cation of these animals and their various relations, in his Families Nat. du Reg. An. 1825. The class of Entozoa is divided into six Orders, founded on the external form of the animals, the internal struc ture not affording as yet any convenient basis of clas sification. The six Orders are, I. Entomoida, 2. Nematoidea, 3. Acanthocephala, 4. Trematoda, 5. Cestoidea, 6. Cystica.