64 LECTURE IV. 



of the pseudo-navicellae, that this is one mode of propagation of the 

 Gregarinae, but the progress of the Navicella to the Gregarina has 

 not been seen. 



The firm nucleus of the Gregarina answers to that of the Polygas- 

 trian ; the cell-membrane to the ciliated integument, and the granular 

 contents to the more-specialised cells which surround the nucleus. 

 The Gregarina may be regarded as a parasitic Monad, and the most 

 simple of the animal kingdom. It differs from the single-celled plant 

 by the contractility of its tissue, and the solubility of its cell-wall by 

 acetic acid. 



I next proceed to consider the internal parasites, which present the 

 characters assigned by Rudolphi to his Cystic Entozoa. 



The name Echinococcus has been given to a cyst resembling the 

 acephalocyst, in the absence of any external organised appendage, 

 but differing in the structure of the innermost layer or "membrana 

 propria," which is very thin and contains many minute clear calca- 

 reous corpuscles, which on the application of a weak acid, liberate 

 bubbles of gas and disappear. The other tissues of this, which 

 should be called the " echinococcocyst," consist, as in the acephalo- 

 cyst, of coagulated albumen, the product probably of the adventi- 

 tious cyst of the organ in which the echinococcocyst is imbedded. 

 From the innermost or proper tissue of the echinococcocyst small 

 pyriform buds protrude, in which are developed one or more minute 

 vesicular organisms, having a head armed with a circle of more or 

 less bent spines {Jig. 24. a.) and in some species also four suckers (ib. b.). 

 These properly represent the genus Echinococcus. As their de- 

 velopment advances, their nursing-cysts become pedunculate, and 

 finally burst or break off, liberating the organisms which then float, or 

 freely swim, in the sero-albuminous fluid of the parent-cyst.* The 

 tissue of the little echinocoeci is highly contractile, and the uncinated 

 head can be retracted within the cavity of the body. 



In the Echinococcus Veterinorum, the species which infests the 

 common domestic animals, the oral spines, so retracted, offer a resem- 

 blance to the cylinder of teeth, which characterises the Nassula 

 {Jig. 14.) and many other Polygastria. The tissue of the body pre- 

 sents a number of clear oval cells or spaces. On examining the fluid 

 of an echinococcocyst from the abdomen of a recently-killed sow, I 

 saw the young echinocoeci moving freely about in it : the anterior end 

 of the body presented a trilobate depression produced by the retraction 

 of the uncinated head, and at the opposite end there was, in some, a 

 small orifice, " from which a granular and glairy substance was 



♦ LVIII., LIX., LX., LXI. 



