NATURE OF REPRODUCTION. 673 



conditions which are favorable for their development ; in the same man- 

 ner as the seeds of plants require, for their germination and growth, a 

 certain kind of soil and a certain supply of warmth and moisture. It is 

 accordingly no more remarkable that Oxyuris vermicularis should in- 

 habit the rectum, and Ascaris lumbricoides the ileum, than that Lobelia 

 inflata should grow only in dry pastures, and Lobelia cardinally by the 

 side of running brooks. The lichens flourish on the exposed surfaces 

 of rocks and stone walls ; while the fungi vegetate in darkness and 

 moisture, on the decaying trunks of dead trees. Yet both these classes 

 of vegetables are well known to be reproduced by generation, from 

 germs which require special conditions for their growth and develop- 

 ment. If the germ of any species, whether animal or vegetable, be de- 

 posited in a locality where these requisite conditions are present, it is 

 developed and comes to maturity ; otherwise not. This accounts fully 

 for the fact that internal parasites, like other living organisms, are con- 

 fined to certain situations by the requirements of their nourishment and 

 growth. 



But in regard to a few of the internal parasites a further difficulty 

 existed, owing to the presence of two peculiaries : first, these particular 

 kinds do not inhabit the open passages or canals of the body, but lie 

 encysted, in the solid substance of the tissues, where there are no visible 

 means of access from without ; and secondly, they are sexless, perform- 

 ing no generative function, and having no progeny of their own ; so that 

 it does not readily appear how they can themselves have been derived 

 from parents. The two kinds of entozoa which have presented this 

 difficulty in the most marked manner, and in which it has been most 

 fully explained by the results of observation and experiment, are those 

 known as Cysticercus cellulose and Trichina spiralis. 



1. Cysticercus celluloses. This is a bladder-shaped parasite of some- 

 what flattened form, about 10 millimetres in diameter, found in the sub- 

 cutaneous and intermuscular connective 



tissue of the pig, where it appears under Fi g- 214. 



the form of whitish specks, giving to 

 the flesh the appearance known as that 

 of "measly pork." Each parasite is 

 enveloped in a perfectly closed c} T st, but 

 the bladder-like body, when extracted, 

 exhibits at one spot a minute depression 



Or involution of its wall. From this CYSTICBRCTTS CELLULOSE, from 



point a slender neck, ending in a ^yS^.iSJ^SSSS 



rounded head, may be extruded by and neck extruded. 2, a. The same, 

 pressure ; after which the animal is seen 

 to consist of a head and neck, termi- 

 nated posteriorly by a dilated, sac-like tail, whence its generic name of 

 cysticercus. Its specific name was derived from its inhabiting the con- 

 nective tissue, formerly known as the " cellular tissue." The head of 

 the parasite, when magnified, shows upon its surface four sucking disks, 



