ON ANIMAL PARASITES. 99 



themselves with this object, I will, therefore, proceed at once to 

 class animal parasites under six heads, as follows : — 



I do not, of course, by any means intend to imply that every 

 parasitic animal can be included in these six classes, but they 

 will be found to include all the principal ones that are as yet 

 known. The others are either not sufficiently understood, or they 

 are not of sufficient interest, to obtain a separate place in a popular 

 classification. Of this number are the Gregarinadcv, the simplest 

 animal forms of which we have any knowledge, and which are 

 parasitic in the cockroach and the earth-worm ; the rare forms of 

 the parasitic Mollusca, viz., the Stylifer, which infests star-fishes 

 and sea-urcliins, and the Synapta digitata, a singidar parasite ol 

 another Echinoderm ; besides a few fishes which prey upon others 

 of their class, and which are the only true parasites, I believe 

 amongst vertebrate animals. 



^o'- 



1. Parasitic Worms. 



In this class are included the tape- worms (Tseniada), the flukes 

 or suctorial-worms (Trematoda), the thorn-headed-worms (Acan- 

 thocephala), the hair-worms (Gordiacea), and the thread- worms 

 or round-worms (Nematoda). All the five orders belong to the 

 Scolicida, and in this formidable class are comprehended, with the 

 exception of the " bots " or larvre of the gad-fiy, all the internal 

 parasites, or at least the chief of those we are acquainted with. 

 They give rise to many serious diseases, both in man and in the 

 lower animals. 



Tape-tvorms (Tfeniada). — These internal parasites in their 

 adult condition, infest the intestines of warm-blooded animals. 

 Some of the worms are said to be 40 or 50 feet, or even more, 

 in length. They are composed of a number of flattened joints 

 which, however, do not really constitute the tape-worm, the 

 true animal being found in the so-called head, while the joints 

 are simply hermaphrodite generative segments, which the head 

 throws off by a process of gemmation or budding. The term 

 "proglottis" is applied to a single segment. The head is 

 very small, and is furnished with a circlet of hooks or suckers, 

 by which the parasite fixes itself to its host, but there is no 

 digestive system, not even a mouth, the nutrition of the animal 

 lieing entirely effected by imbibition. In its development the 



