ANNULOIDA: SCOLECIDA. 159 



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 Taniada 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^NIADA (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. 46) 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 gemma- 

 tion. The " head " (fig. 46, 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; but 

 there is considerable obscurity on this point, and it has been 

 asserted that the nervous system is entirely wanting, or that 

 there is only a single ganglion. The " water-vascular system " 

 consists of a series of long vessels which run down each side 

 of the body, communicating with one another at each articula- 

 tion by means of a transverse vessel, and opening in the last 



