"184 MANUAL OF ZOOLOGY. 



and Rotifera, with a section of the Nematoidca, lead a free 

 existence, and are not parasitic within other animals. 



The Scolecida are defined by the possession of a "water- 

 vascular system," consisting of a " remarkable set of vessels 

 which communicate with the exterior by one or more apertures 

 situated upon the surface of the body, and branch out, more 

 or less extensively, into its substance " (Huxley). No proper 

 vascular apparatus is present, and the nervous system (when 

 present) " consists of one or two closely approximated 

 ganglia." The habits and mode of life of the different mem- 

 bers of the Scolecida are so different, that no other character, 

 save the above, can be predicated which would be common to 

 the entire class, and would not be shared by some other allied 

 division. 



DIVISION I. PLATYELMIA. This section includes those Scole- 

 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. 68) 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," 

 or "nurse," whilst the joints are simply hermaphrodite, 

 generative segments, which the " head " throws off by a 

 process of gemmation. The "head" (fig. 68, 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 



