118 : INVERTEBRATE ANIMALS. 
6. Nematoda (Round-worms and Thread-worms). 
7. Rotifera (Wheel-animalcules). 
OrpeER I. T#NIADA.—In this order are comprised the so- 
called Tapeworms (fig. 50, 5) and the bladder-worms or cystic 
SED ‘ 
SE , 
! Sas Sel 
ASSES |; 
A 
I Boh) 
ie 
GZ 
af 
> 
2 
UIT 
ly, 
Ly 
Fig. so.—Morphology of Tzniada. 1. Ovum containing the embryo in its 
leathery case; 2. A bladder-worm (Cysticercus longicollis), magnified; 3. 
Head of the adult Tea solium, enlarged, showing the suckers and crown of 
hooklets; 4. A single generative joint, enlarged to show the branched ovary (0), 
the generative pore (a), and the water-vascular canals (6); 5. Fragment of 
Tenia solium, showing the generative joints and the alternate arrangement of 
the generative pores. 
worms (fig. 50, 2). These were formerly described as distinct 
groups; but it is now known that the latter are merely the im- 
mature forms of the former. The peculiarity which distin- 
guishes the development of the Z@zzada, and which led to the 
cystic worms being described as distinct animals, is that the 
different stages of growth are always found inhabiting different 
animals or “ hosts.’ If the fully-grown tapeworm is found in 
one animal, then its young form or cystic worm will always be 
found in another. Many animals are infested by tapeworms ; 
but all the leading points of interest in the order will be 
brought out by a consideration of the commonest of the three 
tapeworms to which man is subject—namely, the common tape- 
