54 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



large intestine, especially in children, its head-quarters being the caecum. 

 The female is rather less than 4 an inch long, and the male about half that 

 length (Fig. 16). The eggs, which are laid in an advanced state of develop- 

 ment, are passed out of the body of the host in vast numbers. There is no 

 intermediate host, and infection is easy, while it is an unusually difficult 

 parasite to get rid of, as its numbers are kept up by self-infection. 



Trichina spiralis (Fig. 17) is a minute parasite ( $ i\ inch, $ ^ inch 

 long), the adult sexual stage of which occurs in the intestines of vertebrates, 

 especially rat, pig, and man. The young are born alive, bore into the wall 

 of gut where they enter blood-vessels, and thus reach the muscles within 

 which they encyst. Infection is caused in the human subject by the eating 

 of "trichinized" pork (i.e., pork containing encysted trichina?) which has 

 not been sufficiently cooked. 



CHAPTER V. ANNELIDA (Segmented Worms). 

 8. LUMBRICUS (Earthworm). 



IN this country there are several species of Earthworm, which 

 differ from one another in comparatively minor details. The 

 largest is Lwribricus herculeus. It and the other kinds are found 

 in damp earth, &c., in which they burrow. The variations in 

 size are very considerable. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The body is much elongated, and 

 sub-cylindrical. The hinder part is somewhat flattened from 

 above downwards. As in crawling animals generally the sym- 

 metry is bilateral, and the dorsal surface is readily distinguishable 

 by its dark-reddish colour from the paler ventral surface. The 

 body is segmented, that is, made up of a series of transverse rings 

 or segments (metameres). These are very numerous (as many as 

 150), and vary much in size. They are largest at the anterior 

 end, where also the shallow grooves, of which one (or more) 

 encircles each segment, are most evident. The front end of the 

 body tapers to a blunt point, the apex of which is formed by the 

 small upper Up (prostomium). This overhangs the transversely 

 crescentic mouth which is mainly bounded by the first or mouth- 



