244 MANUAL OF ZOOLOGY. 



interior of various insects during part of their life. The 

 digestive system is imperfect, an anal aperture being univer- 

 sally wanting. In Mermis, the gullet ends in a blind sac ; in 

 Gordius, the digestive tube opens into the body-cavity; and 

 in Sphczrularia, the mouth appears to be wanting. The sexes 

 are in different individuals. In Gordius itself, the embryo is 

 free and aquatic, having a retractile snout armed with hooklets, 

 by means of which it, after a time, bores its way into the 

 tissues of some water-insect, in which it encysts itself. The 

 sexually-mature worms are found in the interior of Orthoptera 

 or Neuroptera; but they leave their hosts and betake them- 

 selves again to an aquatic existence for the purpose of laying 

 their eggs. The 'adult Mermis is found principally in Lepidop- 

 tera; whilst Spharularia inhabits the body-cavity of Bumble- 

 bees. A form of the Gordiacea has also been found at great 

 depths in the ocean, coiled up beneath the carapace of shrimps 

 (Willemoes-Suhm). 



ORDER III. NEMATODA (or Nematofdea). Cylindrical vermi- 

 form Scolecids, sometimes parasitic, sometimes free ; integument 

 not ciliated ; a well-developed alimentary canal, with a mouth 

 and anus, suspended freely in a body-cavity ; sexes distinct, or 

 rarely united. 



The Nematoda comprise the so-called " Thread-worms " and 

 " Round-worms," and, as their various names imply, possess a 

 rounded and worm-shaped body (fig. 120, a), sometimes of 

 great length. The cuticle is chitinous and porous ; and there 

 is generally a distinct annulation, though no true segmentation 

 exists. The alimentary system is well developed, the mouth 

 being anterior, and usually furnished with papillae (fig. 120, c). 

 The gullet opens into a muscular stomach, from which an in- 

 testine conducts to a ventrally or terminally placed anus ; the 

 whole digestive tube being freely suspended in a body-cavity, 

 which is filled with a sparsely corpusculated fluid. There is a 

 water-vascular system, composed of lateral tubes, which open 

 on the surface by a ventrally - placed pore. The nervous 

 system is in the form of a ganglionic ring, surrounding the 

 gullet, and sending filaments backwards and forwards. The 

 sexes are mostly distinct, the external openings of the repro- 

 ductive organs being placed near the anus. The males are 

 usually less frequently met with and of smaller size than the 

 females, and they possess a single or double spicular penis 

 (fig. 120, b). Metamorphosis may or may not occur during 

 development. 



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

 inhabiting the alimentary canal, the pulmonary tubes, or the 



