448 



CHARACTERS OF INVERTEBRATE ANIMALS 



lines. As a result of this, the underlying muscle, which constitutes 

 a sort of longitudinal layer, is broken up into four sections. 



The digestive organs (fig. 276) consist of a narrow tube 

 running straight through the body, beginning with a thick-walled 

 gullet which passes into a thin-walled intestine. 

 Between the gut and body-wall is a body-cavity 

 containing fluid, but there is nothing else to which 

 the name of circulatory system can be applied. 

 There are no special breathing organs, and the 

 excretory organs are quite unlike those of a seg- 

 mented worm (p. 428), consisting as they do of 

 a narrow tube running in each lateral line, and 

 joining with its fellow in front to open by a minute 

 pore on the under surface. The nervous system. 

 again is in many respects unlike that of a seg- 

 mented worm (p. 428). It is closely connected 

 with the skin, and its most conspicuous portions 

 consist of a ring not far behind the mouth, from 

 which nerves are given off both fore and aft. 

 Those in front supply the lips, and of the others 

 the largest are a dorsal and a ventral nerve which 

 traverse respectively the dorsal and ventral lines. 



A common example of free -living Thread- 

 Worms is the little Vinegar Eel {Anguillula aceti), 

 common in paste and weak vinegar. Another 

 curious creature, representing a group differing in 

 many respects from the one containing Ascaris and 

 Anguillula, is the Gordian-Worm {Gordius). This 

 is a very slender black worm, often to be seen in pools or puddles, 

 and the name has reference to the fact that a number of them 

 may often be found twined together into a complicated tangle. 

 In early life the Gordian-Worm is parasitic in an insect, leaving 

 its host, however, when it becomes adult, and as large numbers do 

 this about the same time, their appearance in public is somewhat 

 sudden. Hence the rustic belief, once prevalent, that they have 

 been rained down from the sky, a common explanation of the 

 rapid appearance in any quantity of all sorts of small animals. 

 Another curious popular idea, suggested by the appearance of the 

 worms, is that they are horse-hairs into which the breath of life 

 has mysteriously entered. 



Fig. 276. — Round- Worm 

 {^Ascaris) 



A, Side view ; B, front 

 end of body, much en- 

 larged to show mouth 

 and lips; INT. AP., in- 

 testinal aperture. 



