MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 193 



Class Chaetognatha (ke tog'na tha — horse's mane, jaw). — These 

 small marine worms are often called arrowworms, and they are well 

 adapted to livmg at the surface of the ocean. Horizontal fins sup- 

 port the animal at the surface and also make it possible for it to 

 move about rapidly. The prehensile mouth with its bristles have given 

 the animal the name of "bristle jaws" in addition to other names. 

 The body is divided into three divisions : head, trunk, and tail. These 

 are separated by septa and the coelomic cavity is separated into right 

 and left cavities by a longitudinal mesentery. 



Internally is a tubelike intestine which extends from the mouth 

 at the anterior, to the anus located near the base of the caudal fin 

 or tail. The nervous system consists largely of a supraesophageal 

 ganglion or brain, ventral ganglion, branch nerves, two eyespots, 

 and other sensory organs. These animals are lacking in circulatory 

 or excretory structure. 



Each individual is capable of producing both ova and sperma- 

 tozoa, that is, the hermaphroditic condition prevails. The ovaries 

 are located in the posterior portion of the body cavity and the 

 mature ova are carried to the exterior by an oviduct on each side. 

 The testes are located in the cavity of the tail portion. The sper- 

 matozoa are discharged into this cavity and delivered to the ex- 

 terior by a pair of slender vasa deferentia or sperm ducts, which 

 enlarge to become seminal vesicles near the aperture. The fertilized 

 ova become small adults without a typical ciliated larval stage. 

 Sagitta is the best known genus of the group. 



