ENTOZOA. 59 



elongated, and the sexes in distinct individuals. The Trematoda, in 

 which the head is unarmed and has a suctorious foramen, the body 

 rounded or flattened, and generally one or more suctorious cavities 

 for adhesion, and in which the organs of both sexes are in the same 

 individual. The Cestoidea, in which the body is elongated, flattened, 

 and generally articulated : the head, variously organised, generally 

 provided with suctorious cavities, sometimes armed with a coronet of 

 hooks, sometimes with four unarmed or uncinated tentacles: both 

 kinds of generative organs are combined in the same individual. 

 Lastly, the order of Cystica, in which the body is rounded or flattened, 

 and terminates posteriorly in a cyst, which is sometimes common to 

 many individuals : the head is provided with suctorious cavities, 

 and with a circle of booklets, or with four unarmed or uncinated ten- 

 tacles. No distinct generative organs are developed in the cystic 

 Entozoa, and there is good evidence that most, if not all, are larvae of 

 a higher order. 



The anatomy of the Entozoa is so distinct in each of these orders 

 that I shall describe it successively in a few typical species, selecting 

 more especially for demonstration those which infest the human body ; 

 and which chiefly concern the medical practitioner. 



In this category the common pathological product called hydatid, and 

 " Acephalocyst" by Laennec, is by many received, and ought not, per- 

 haps, in this place to be omitted. The acephalocyst {Jig. 23, b.) consists 



of a sub-globular or oval vesicle filled 

 with fluid. Sometimes suspended freely 

 in the fluid of a cyst of the surrounding 

 condensed cellular tissue (a) ; some- 

 times attached to such a cyst ; deve- 

 loping smaller acephalocysts (c), which 

 are discharged from the outer or the 

 inner surface of the parent cyst. 

 Acephalocyst. These accphalocysts vary from the 



size of a pea to that of a child's head. In the larger ones the wall of 

 the cyst has a distinctly laminated texture. They are of a pearly 

 whiteness, without fibrous structure, elastic, spurting out their fluid 

 when punctured. Their tissue is composed chiefly of a substance 

 closely analogous to albumen, but differing by its solubility in hydro- 

 chloric acid; and also of another peculiar substance analogous to 

 mucus.* The fluid of the acephalocyst contains a small quantity of 

 albumen with some salts, including muriate of soda, and a large pro- 

 portion of gelatin. 



The tunic of the acephalocyst is usually studded with more or less 



* Ln. 



