INDIVIDUALITY OF THE JOINTS. 



275 



We would not, however, conceal that there are, on the other hand, 

 certain tape-worms in which the sexual animals are never isolated, 

 and some indeed in which the jointing 

 is so incomplete that only an in- 

 distinct constriction (Tricenophorus) or 

 merely successive sets of generative 

 organs (Ligula) suggest the composite 

 nature of the body. If we compare 

 this state of matters with what we find 

 in other composite animals, we shall 

 not be led into error ; it only proves 

 that the independence of the united 

 individuals, or, if one prefer it, the 

 individualisation of the joints, is of 

 different degrees in different cases. 



Even the external characteristics of 

 the tape-worms give us some standard 

 in judging of these degrees of indi- 

 vidualisation. The earlier and the 

 more sharply the individual joints are 

 marked off from one another, and the 

 looser their mutual association, the 

 greater will be their independence 

 in a biological sense. And thus it is 

 the typical forms of Tcenia, with their 

 narrow joints, like the seed of a 

 melon (Fig. 131), which most evidently 

 exhibit the polyzootic nature of these 

 animals. The joints separate singly, 

 and then in their movements and 

 behaviour appear as perfect, indepen- 

 dent individuals. In the broad, short 

 joints of the Bothriocepliali (Fig. 137), 

 this independence is not so marked, for 

 they are liberated, not singly, but in 

 groups of varying number. In the unjoin ted forms there is, of course, 

 no regular liberation. The individual parts are here subordinated to 

 the whole, both anatomically and biologically. 



The forms hitherto mentioned do not, indeed, represent the highest 

 degree of centralisation possible to the Cestodes, for there are tape- 

 worms, as has been previously pointed out (p. 106), where the body 

 is destitute of any kind of joint. There is a short proglottis like 

 a flat-worm which contains only a simple generative organ, and is 



FIG. 137. Bothriocephalus latus. 



