142 MANUAL OF ZOOLOGY. 



cida which possess a more or less flattened body, usually some- 

 what ovate in shape, and not exhibiting anything like distinct 

 segmentation. The division includes two parasitic orders 

 the Tceniada and the Trematoda and one non-parasitic order, 

 viz. the Turbellaria. A sub-order, however, of this last, the 

 Nemertidce, does not conform to the above definition ; but 

 their other characters are such as to forbid their separation. 



ORDER I. T^ENIADA (Cestoidea). This order comprises the 

 internal parasites, called Tape-worms (Cestoid worms), and 

 the old order of the * Cystic worms ' (Cystica) ; the latter 

 being now known to be merely immature forms of the Tape- 

 worms. 



In their mature condition, the Tceniada (see fig. 39) are 

 always found inhabiting the alimentary canal of some warm- 

 blooded vertebrate animal ; and they are distinguished by 

 their great length, and by being composed of a number of 

 flattened joints, or articulations. These joints are not, how- 

 ever, an example of true segmentation, nor do they really 

 constitute the Tape-worm ; the true animal being found in 

 the small, rounded, anterior extremity, the so-called ' head,' 

 whilst the joints are simply hermaphrodite, generative seg- 

 ments, which the * head' throws off by a process of gemmation. 

 The 'head' (fig. 39, 3), which constitutes the real Tape- 

 worm, is a minute, rounded body, which is furnished with a 

 circlet of hooks, or suckers, or both, whereby the parasite is 

 enabled to maintain its hold upon the mucous membrane of 

 the intestines of its host. No digestive organs of any kind are 

 present, not even a mouth ; and the nutrition of the animal is 

 entirely effected by imbibition. The nervous system consists 

 of two small ganglia, which send filaments backwards. The 

 ' water- vascular system ' consists of a series of long vessels 

 which run clown each side of the body, communicating with 

 one another at each articulation by means of a transverse ves- 

 sel, and opening in the last joint into a contractile vesicle. 

 It, thus, appears that all the joints are organically connected 

 together. Whilst the ' head ' constitutes the real animal, it, 

 nevertheless, contains no reproductive organs, and these are 

 developed in the joints or segments (fig. 39, 4), which are 

 produced from the head posteriorly by budding. After the 

 first joint, each new segment is intercalated between the head 

 and the segment, or segments, already formed ; so that the 

 joints nearest the head are those latest formed, and those furthest 

 from the head are the most mature. Each segment, wheii 

 mature, contains both male and female organs of generation, 

 and is, therefore, sexually perfect. To such a single segment 

 the term * proglottis ' is applied, from its resemblance in shape 



