250 



NATURAL HISTORY. 



or they are filiform, and nai-rowed at each end, and furnished with papillae, or stylets, on the anterior 

 extremity. The sexes are separate. The class is divided into two orders, the Acanthocephala and the 

 Kematoidea, and in both there are no rudimentary organs of locomotion, such as false feet, and it is only 

 in rare instances that setae capable of moving are found. Usually the skin is thick and the muscular 

 system within it is highly developed, so that these worms wriggle, twist, and move in a serpentine 

 manner with great vivacity and persistence. Within the muscular layer is the visceral cavity which 

 contains the blood and the digestive and reproductive organs. There are no special organs of circula- 

 tion and respiration, but there is a nervous system, and there is a tactile power in the front of the 

 body, especially when there are papillae developed there. Simple eye-spots and eyes have only been 

 noticed in the non-parasitic kinds. Great diversity exists in the shape of the digestive oi-gans, and in 

 one order they are absolutely deficient. The excretory organs exist, and are various in their shape 

 and distribution. In almost every instance the sexes are in different individuals, and the egg may 

 produce a worm like the parent, or a form which has to undergo metamorphosis of very strange 

 kinds, one part of the transformation taking place inside one animal, and the other in a second 

 and different kind of unwilling animal host. The parasitism of most is constant, but in some a 

 host is frequented at one time only, of the life of the parasite. The parasitism is of a nature 

 deserving the name, and the worms live in their unwilling host, and exist by absorbing its juices. 



The Acanthocephala, or Thorn-headed Worms, have a genus whose name is explanatory of the 

 principal peculiarity of the order.* They have a projecting trunk or proboscis which is armed with 



hooks ; the body is ovoid and oblong or cylindrical, and 

 has neither mouth nor digestive organs. The trunk is 

 used to fix the worm, or to enable it to penetrate the coats. 

 of the intestine of its host. The nervous system is com- 

 posed of a ganglion with large cells, which give forth a 

 nerve to the proboscis and another to the body ; but 

 there are no sense nerves. The species of Echinorhynchus. 

 are frequently parasitic within Invertebrata in their first 

 stage of metamorphosis, and within Vertebrata in their 

 second, and become perfect there. Thus eggs containing 

 embryos are excluded, and these escape in the form of 

 little elongate bodies armed in front with temporary 

 hooks. They live in the water free, and are swallowed 

 by, or penetrate from without, through the tissues into 

 the digestive organs of small Amphipod Crustacea. 

 After a while they cling to the tissues of the stomach 

 and intestine of the Crustacea, and penetrate them, 



enlarged) -, c, EGGS OF DO. (enlarged). [After Busk.] hooks and undergo a metamorphosis, becoming round or 



elongate things which might be called nymphs. If the 



Crustacean should happen to be swallowed by a bird or fish, the Echinorhynchus is not killed, but it 

 escapes from the prey and fixes on to the mucous membrane of the digestive organs of the swallower,. 

 and then attains its perfect form, living by taking in, through its skin, the nourishing juices of the food 

 of its host. This process of development may be considered one of alternation of generation. Examples 

 are very common. Thus one kind of Echinorhynchus affects the Water Flea (Gammarus pulex), and 

 this is swallowed by the fresh- water fish, and another kind gets into the food of water- fowl, and 

 becomes parasitic within it. 



ORDER NEMATOIDEA.f 



The Thread Worm group are round worms, with a long, fusiform, or filiform body. They are 

 mostly pai-asitic, and usually have a mouth, a swollen gullet, and a straight digestive canal. The 

 cylindrical body, generally very long for the width, has papillae on the front of the body around the 

 mouth, and sometimes sharp pricks and hooks, or a style in the interior of the buccal cavity. There 

 is often a very muscular, dilated pharynx, and there may be a granular substance in the spaces left 

 * Echinorhynchus. t Greek, ncma, a thread. 



