no INVERTEBRATE ANIMALS. 



The common tape-worm is found inhabiting the intestines of 

 man, one only being generally present in the same individual. 

 In shape (Fig. 39, 5) it is an extremely elongated, flattened, 

 tape-like body, many feet in length, and composed of a num- 

 ber of flattened joints (Fig. 39, 4) all loosely united to one 

 another. At one extremity the joints become much smaller 

 and narrower, till ultimately a point is reached where the 

 organism is firmly fixed to the mucous membrane of the in- 

 testine by means of a minute rounded head (Fig. 39, 3). 

 The organs by which attachment is effected are, in this spe- 

 cies, a crown of recurved hooks and four suckers. The head 

 is in reality the true animal, and all the long, jointed, tape- 

 like body which follows this, is really produced by a process 

 of budding from the head. The head contains no repro- 

 ductive organs, and is not furnished with a mouth or diges- 

 tive organs of any kind, its nutrition being entirely effected 

 by imbibition of the nutritive fluids elaborated by its host. A 

 nervous system, in the form of one or two ganglia, sending 

 filaments backward, is said to be present ; but there is soine 

 doubt on this point. The water-vascular system (Fig. 39, 4) 

 consists of two long vessels which run down each side of the 

 body and communicate at each articulation by a transverse 

 vessel, the whole opening in the last joint into a contractile 

 vesicle. Each joint is sexually perfect, or hermaphrodite, 

 containing both male and female reproductive organs (Fig. 

 39, 4), which open on the surface by a small raised aperture, 

 the "generative pore." Almost the whole of each of the 

 mature joints is filled up bya much-branched ovary. As the 

 head is the true animal, and the numerous joints are only pro- 

 duced by budding, it follows that the entire organism is to be 

 regarded as a kind of colony, constituted by a single sexless 

 zo6id or " nurse," and numerous sexual zooids, produced by 

 budding from the former. 



The process of development — that is to say, the process 

 by which this composite organism, commonly known as the 

 tape-worm, is produced — is a very remarkable one, and is 

 briefly as follows: Each generative segment or joint, as al- 

 ready said, is hermaphrodite, and contains innumerable ova. 

 These eggs, however, cannot be developed within the body 

 of the animal infested by the tape-worm itself, but they are 

 compelled to gain access to the body of some different species 

 of animal, if development is to proceed. To secure this end, 

 the mature joints of the colony break off, and are expelled 

 from the alimentary canal of the host. The joints thus ex- 



