CHARACTERISTICS OF THE CYSTICEECOIDS. 655 



If we leave out of account the Cysticerci (p. 343) found 'in the 

 parenchymatous tissue of birds {Piestocystis, Dies.), which in many 

 respects connect the ordinary bladder- worms with those now under 

 discussion, we may say that only cold-blooded animals harbour the 

 Cysticercoids. The majority occur in the Arthropoda, and especially 

 in insects, but also in fish, worms, and snails. In marine animals the 

 Cysticercoids are extremely rare ; we know as yet only a single form 

 (from a Pteropod), but this readily harmonises with the fact that 

 the adult Tseniadse occur very rarely in marine animals. The great 

 majority inhabit terrestrial and fresh water animals, especially birds 

 and mammals, and chiefly those which feed on insects and vegetable 

 food. Both classes of animals get their parasites from their food, the 

 one by directly eating the intermediate hosts, the other by the 

 accidental swallowing of the same; Man, mostly in his youth, may 

 be infected in either of these ways. 



We have already noted the numerous variations in the external 

 appearance and internal structure of the Cysticercoid tape-worms. 

 We find among them species with many, and species with few joints, 

 some small and short, others long and broad, some, indeed, the longest 

 tape- worms we know,^ some armed, others bookless. In many of the 

 latter the head is as large as, or even larger than, that of Tcenia 

 saginata, while others, especially among the armed forms, are less 

 developed in this respect. Similarly the hooks show in number, size, 

 and arrangement the most manifold variations, much more striking 

 than in the cystic worms. Generally the development of the attaching 

 apparatus is less powerful. With, too, this agrees the formation 

 of the rostellum, whose muscular walls enclose a spacious cavity, 

 which is all the more striking since the early lenticular form has 

 been exchanged for an ovoid or cylindrical one. In the last instance 

 the apex often forms on the protrusion of the rostellum a more or less 

 long and lank proboscidiform projection. 



The adult proglottides are usually short, much shorter, indeed, 

 than they are broad, and are more firmly connected with one 

 another than in the cystic tape-worms, so that in expulsion long 

 stretches rather than single joints are set free. The uterus has 

 usually the form of a wide cavity with more or less numerous boss- 

 like or pouch-like diverticula. Here and there we find the dendritic 

 ramifications of the larger cystic tape-worms, with this difference, 

 however, that the main stem has, in correspondence with the shape of 

 the joints, a transverse instead of a longitudinal course. In other 

 cases the diverticula become isolated as they are filled with eggs 



^ The TcBnia expansa of the ruminants is sometimea, accorjJing to Goze and others, 

 over 100 feet long. It has ^[^^Jt^^jijg-p y^-|b^^y^Uke Dipylidium. 



