CHAPTER XYII 



PHYLUM CHAETOGNATHA 



THE Chaetognatha (x a ""?7, hair; yvdOos, jaw) are small cylindrical 

 animals which swim at the surface of the sea. r rhe name is sug- 

 gested by the circumstance that at the sides of the mouth are two 

 rows of curved movable bristles by means of which they seize their 

 prey (e, Fig. 180). 



The body has a small rounded head in front and tapers to a tail 

 posteriorly ; it is provided with one or two pairs of flat, lateral 

 expansions termed fins ; the general shape resembles that of a 

 torpedo, if we leave the head out of account. It has also been 

 compared to an arrow, and the popular term " Arrow-worm " has 

 been applied to these animals. The head is surrounded by a fold 

 of the skin forming a hood which is most prominent at the sides 

 and dorsal surface. Within the hood the head bears on each side 

 a row of sickle-like hooks whose points when at rest converge 

 around the mouth, but are capable of being widely divaricated. 

 The head also bears one or more rows of stout spines whose 

 number and arrangement are of importance for the system of 

 classification (/, Fig. 180). 



The coelom is well developed and contains a fluid in which cells 

 float. In strictness there are three pairs of coelomic sacs separated 

 from one another by transverse and longitudinal partitions. In 

 the head the coelomic space is practically obliterated by the great 

 development from its walls of the muscles which move the hooks. 

 The coelom of the trunk and tail is further divided into right and 

 left halves by a vertical mesentery, which in the trunk region 

 supports the alimentary canal (Fig. 181). This mesentery is 

 pierced by numerous small holes. 



The skin is covered by an epithelium more than one layer thick, 

 some of the cells of which are modified to form sense-organs, while 



