ANNULOIDA: SCOLECIDA. 



I6 9 



developed within a hooked embryo, from which it is se- 

 condarily produced. 



The " Thorn - headed worms " include some of the most 

 formidable parasites with which we are acquainted. The 

 Echinorhynchus (fig. 49) is found in the intestinal canal of 

 many vertebrate animals, especially of birds and fishes. 



ORDER II. GORDIACEA. The Gordiacea, 

 or " Hair-worms," are thread-like parasites, 

 which in the earlier stages of their existence 

 inhabit the bodies of various insects, chiefly 

 of beetles and grasshoppers. They possess 

 a mouth and alimentary canal, but they are 

 not provided with a distinct anal aperture. 

 In Gordius itself the gullet is said to open 

 directly into the body-cavity. The sexes 

 are distinct, and they leave the bodies of 

 the insects which they infest in order to 

 breed; subsequently depositing their ova 

 in long chains, either in water or in some 

 moist situation. 



In form the Gordiacea are singularly like 

 hairs, and they often attain a length many 

 times greater than that of the insect which 

 harbours them. 



ORDER III. NEMATODA (or Nematoidea). 

 The Nematoda " Thread - worms " or 

 " Round - worms " are of an elongated and cylindrical 

 shape ; and are often, though by no means always, parasitic 

 in the interior of other animals. They possess a distinct 

 mouth and an alimentary canal which is freely suspended in 

 an abdominal cavity, and terminates posteriorly in a distinct 

 anus. They also possess a system of canals, in some cases 

 contractile, which open externally near the anterior part of 

 the body, on the ventral surface, or by lateral pores, and are 

 probably homologous with the water-vascular system of the 

 Tceniada and Trematoda. The sexes are distinct, and the 

 males are usually less frequently met with, and of smaller 

 size, than the females. The nervous system is mostly well 

 developed, and is in the form of a ganglionic ring, surround- 

 ing the oesophagus, and sending filaments backwards. 



As before said, most of the Nematoda are internal parasites, 

 inhabiting the alimentary canal, the pulmonary tubes, or the 

 areolar tissue, in man and in many other vertebrate animals ; 

 but a large section of the order are of a permanently free habit 

 of existence. 



. Acanthoce- 

 . ; a Echinor- 

 hynchus gigas, nat. 

 size ; a' The head of 

 the same magnified. 



