This order comprehends only the genus Echi norhynchus, of which there are nearly a hundred species. They are named Acanthocephala, (from axavAct spina,) from the reverted hooks disposed in series around their retractile proboscis. The body has the lengthened, slender, cylindrical form and elastic consistence of the preceding order, and the sexes are placed on separate individuals. The common Echinorhynchus gigas, which occurs in the small intestines of the wild and domestic hog, is one of the largest intestinal worms, and its ana tomy has lately been the subject of an interesting memoir by Jules Cloquet. It possesses distinct transverse and longitudinal muscular fibres within the elastic integument, the nerves are doubtful. The anterior part of the body. which is the broad est, is terminated by a spherical perforated canal, but the food appears to be introduced both by the proboscis and by superficial pores. The sexes are separate. The male organs consist of two curved cylindrical testicles, two vasa differentia, which unite and terminate in a single vesicula seminalis, and a long cylindrical straight tubular and exser tile penis. The female organs are two long wide cylindrical ovaria, which fill nearly the whole of the body, and a single oviduct.
The worms of this order named Trematoda by Rudolphi (front foramen), are chiefly dis tinguished by their having one or more simple pores adapted for sucking. They are generally of an irregular form, of a soft texture, very minute, and with a depressed body. There is sometimes a single pore, sometimes there are several, placed at the anterior or on the lower part of the body, and not provided with recurved spines for piercing. To this order belong the genera.
The name given to this order by Rudolphi is derived from the Greek word erh; tinea, and is ap plied to those entozoa which have an elongated, flat, tapelike form of body, as the common Tania solium found in the human intestines. The body is depressed, softer than the filiform species, and generally divided by transverse depressions pre senting the appearance of a series of articulations, like the annulose animals. The head is generally provided with two or four apertures, adapted fur sucking, and in a few the head appears simply lobed. The sexes appear to be united in the same individuals. Zeder considered the sexes as separate in some of the species where other naturalists have been unable to detect that distinction. The genera belonging to this order are This order comprehends the simplest known Entozoa, in which we observe no distinct internal organs for digestion or generation, and no distinc tion of sex has been observed in any of the species. The body is generally short, slender, cylindrical, or depressed, and terminated posteriorly by a wide vesicle. Some arc found enclosed solitary in shut cysts, to which they do not in any part adhere. The vesicle which terminates the body posteriorly is generally filled with a transparent thin fluid, and the anterior extremity of the animal is provided with sucking pores, and a mouth surrounded with sharp recurved teeth for piercing. The different kinds of Hydatids which abound so much in the digestive organs of ruminating and Pachydoma.
tons animals, are examples of the Entozoa belong ing to this order. Rudolphi comprehends under the order Cystica (from xurir vesica) the four fol lowing genera, viz.
1. Anthocephalus. 3. Coenurus.
2. Cysticercus. 4. Echinoccus.
The animals of this numerous and diversified class are not parasitic like those we have last consider ed, they often acquire a great size, and exhibit a complicated external form, with a complex in ternal organization. They are aquatic animals residing mostly in the sea, some move freely to and fro through that element, others only creep along the bottom, and some remain fixed to sub marine bodies. They are particularly distinguish ed by the radiated disposition of the whole body, or of some of its parts. Several present distinct nerves and ganglia, disposed in a circular manner around the entrance to the stomach. The stomach sometimes terminates in an intestinal canal con nected by a kind of mesentery to the parietes of the body, and ending in a separate anus, as in the Echinus and Spatangus. In others, as the Asterias, the stomach has but one aperture, and sends out numerous ramified caeca to the different divisions of the body. In some of the Medusaria the stomach has several external orifices, and numerous pas sages extends from its parietes through the sub stance of the body. In some of the radiata ves sels for a partial circulation are observed forming plexuses round the stomach, and respiratory or gans in form of minute absorbing tubes, are some times seen spread over the surface. No distinction of sex is observed in these animals or organs for impregnation, and the ovaria which they possess appear destined to an ovo-ge.mmiparous kind of ge neration. A few of these animals afford a palatable nutriment, as the ovaria of the Echinus esculentus. They constitute the food of the largest animals of the globe—the whales. They produce the lumi nosity of the sea by night, and from their abund ance they often change its colour by day. They afford a rich manure, and are often collected in immense quantities for this purpose in France, and in this country among the exuvia of the sea, to be spread upon the land. For the illustration of this interesting class of animals we are chiefly in debted to the labours of Leske and Klein for the Echinoida, Linck and Teidemann for the Asteroida, Reaumur and Dicquemare for the Anthoida, Muller, Forskael, Basterus, Cuvicr, Peron, Lesueur, and Rosenthal for the Medusaria. And the industry of Buguiere and Lamarck have been advantageous ly directed to this class. Cuvier has distributed these animals into two classes, Echinodermata and Acalepha, the former, instituted by Bruguierd for those species, which, like the Asterias and Echi nus, are covered externally with calcareous spines; and the latter class, instituted by Cuvier. for the naked Radiata, which, like the Meduste and Acti nia, have a stinging quality when they touch the naked hand in their living state. Lamarck has placed them under the same class Radiata. The orders of this class are natural families, which de rive their names from those of their typical genera. They are five in number, viz. Holothurida, Echi noida, Asteroida, Anthoida and Medusaria.