164 CLASS V. 
ledge it to be unnatural. Rupo.rui, to whose investigation this 
part of Zoology is so greatly indebted, compares the class of the 
intestinal worms to a Fauna, a collection of animals that live in a 
certain region. Their country is the living body of other animals. 
Recent observations have, however, taught us that some species in 
the cycle of their developments, leave their country for a time. The 
radial type, which we observed in the former classes, particularly 
in the Polyps and Acalephs, is here wanting. All Hntozoa approxi- 
mate to the elongated form of the articulate worms, although in 
some of them that form, on account of a bladder to which they are 
affixed by their extremity, is not at first sight apparent. Special 
respiratory organs are wanting. Some are entirely without sex, 
and are propagated by gemmation; others are bisexual; in others 
the sexes are distinct. 
We must premise something as to the division of the Hntozoa. 
RvDOLPHI adopts five principal sections, which had been already 
distinguished by Gorrze and ZEDER before his time. The first is 
that of Cystic worms (Cystica from xiors, a bladder). The body is 
compressed or cylindrical, the posterior part of it passing into a 
bladder; sometimes many such entozoa are fixed on one bladder. 
The head has suckers, a circlet of hooks, or four proboscides armed 
with hooks. No genital organs have been discovered; nor any 
intestinal canal. [These have been shewn to be imperfectly deve- 
loped Teniev.] The second division contains the Tape-worms 
(Cestotdea, from xeordés, a girdle, a band, efdos, form). They have an 
elongated, compressed or flat body that is mostly jomted. The head 
has suckers; all the individuals are bisexual; an intestinal canal is 
wanting. The third division is formed by the Suctorial Worms 
(Trematodes, rpnnarédns, perforate). ‘The body is soft, compressed 
or roundish, and provided with one or more suctorial pores. The 
intestinal canal has in general only one opening, and is usually 
branched; it les in the tissue (parenchyma) of the body, not in a 
free space. The genital organs of both sexes are united in the 
same individual. The fourth division is that of the Thorn-headed 
Worms (Acanthocephala, from dxavéa, a thorn, and xepady, head) ; 
they have a cylindrical, pouch-shaped body. The sexes are 
distinct ; an intestinal canal is wanting. The fifth division includes 
those intestinal worms which have an intestinal canal, with both 
mouth and anus, suspended freely in the cavity of the body. The 
