1 78 Introduction to Animal Morphology. 



Two orders are included : — 



I . Gordiacea [wnSiebold) — long, thread-like, elastic, 

 aproctous, with a firm, fine cuticle, and a faintly- 

 fibrous basement, covering a layer of longitudinal 

 fibres (which are flattened bands, with their flat sur- 

 faces in contact with each other), only interrupted 

 along the median line. The water-vascular system is 

 absent (Gordius), or represented by a row of cells 

 (Mermis). The short oesophagus passes from the 

 mouth to end in a bulb (Mermis), or in an intestine 

 (Gordius) opening into the body cavity. The nervous 

 system is feeble, but of the nematode type. The 

 sexual organs consist of simple tubes, developed in 

 the peri-enteric connective tissue, at the expense of 

 the vanishing digestive organs, and opening poste- 

 riorly (the female opening may be central in Mermis). 

 They are parasitic in insects for some part of their 

 life, free at other times ; when mature, they live mostly 

 in water or mud, where they reproduce, and their 

 young either return to their insect hosts (Mermis) or 

 undergo metamorphosis. They reproduce in large 

 numbers, and so suddenly do the young of M. albicans 

 appear in favourable seasons, as to give rise to the 

 belief that they have fallen with rain. The larvae 

 have one or more cephalic boring spines, sometimes 

 (Gordius) an armature of recurved hooks. 



There are three families and about thirty species : — 

 I. Gordiidae — yellowish brown, or black, with an intestine ; 

 head not papillose ; females entire or forked at the tail ; 

 uterus one-horned ; males forked, with the spicule-less sexual 

 opening in the fork ; eggs pear-shaped. Gordius begins life 

 free, with a retractile spiny proboscis and a transversely 

 wrinkled body ; it enters some aquatic insect, and becomes 



