132 ZOOLOGY. 



changes take place. The numerous transverse folds in the 

 body disappear, and it becomes twice as long as before, its 

 head-armature disappears, the body becomes swollen, milky, 

 and pulpy. It remains immovable in the water for a vari- 

 able period, and then increases in size ; the integument grows 

 harder, and when about two inches long it turns brown and 

 begins to move. Most hair-worms live in ground beetles 

 and locusts, twining round the intestines of their host, 

 finally passing out of the anus. They are often seen in 

 fresh water pools, twisted into knots, whence their name 

 Gordius. They sometimes occur in horse-troughs, whence 

 they are supposed by the ignorant to be transformed horse- 

 hairs. 



Order 3. Chcetognathi. This group is represented by a 

 single genus, Sagitta, which, from the singularities in its form 

 and structure, has by different authors been referred to the 

 Crustacea, the Mollusca and even the Vertebrates. Its de- 

 velopment and structure show that it is closely allied to the 

 Nematode worms. It is about two centimetres (nearly one 

 half inch) in length, and is found swimming at the surface 

 of the ocean in different parts of the world. The lateral and 

 caudal fin-like expansions of the skin of the end of the 

 body gives it a fish-like appearance. There is a well-defined 

 head, with several curved spines on each side of the mouth, 

 which serve as jaws ; besides these, at the sides of the head 

 are four sets of short, strong spines. In the young Sagitta 

 there are also a few pairs of lateral spines behind the head, 

 but these afterwards disappear. The alimentary canal forms 

 a straight tube terminating in a ventral opening near the 

 posterior fourth of the body. The nervous system consists 

 of a brain from which two nerves are distributed to the eyes, 

 and two lateral nerves pass backward to a large ventral gan- 

 glion lying near the middle of the body, from which two 

 threads pass backwards. The sexes are united in the same 

 individual, the two long tubular ovaries communicating by 

 two long ciliated oviducts, each with a separate outlet at the 

 base of the tail. Behind the ovaries and anus are two cham- 

 bers in which the spermatic particles are developed from mass- 

 es of cells floating freely in the perivisceral fluid, and escap- 



