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. Chetognathi.—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 perivisccral fluid, and escap- 
