146 



INVERTEBRATE ZOOLOGY 



transverse septum, and a longitudinal mesentery divides each 

 coelomic cavity into a right and a left part. From the sides of 

 the head (Fig. 74) there extend a series of bristles which act as 

 jaws in seizing prey, hence the name Chaetognathi, or bristle 

 jaws. In preserved specimens, these jaws are frequently folded 

 close against the sides of the head. 



Swift movements are produced by muscular contractions of 

 the body and are directed by fins which occur as outgrowths 

 of the body wall. These are a fan-shaped tail fin and one or 

 two pairs of lateral fins on the trunk. Some species undergo 



marked migrations between 

 the surface and deeper water 

 twice daily. In Southern 

 California one species be- 

 comes most abundant at the 

 surface of the ocean just 

 after sunrise and again just 

 after sunset. The central 

 nervous system consists of a 

 cerebral ganglion on the 

 dorsal surface of the esopha- 

 gus which is connected with 

 a ventral ganglion near the 

 middle of the trunk by two 

 long esophageal connectives. 

 A pair of eyes lie close behind 

 the brain with which they are connected by a pair of nerves. 

 Papillae scattered over the surface of the body probably 

 have a tactile function. A peculiarly modified region on 

 the dorsal surface just behind the head has been interpreted 

 as olfactory. 



The individuals are hermaphroditic. Ovaries occur in the 

 posterior part of the trunk cavity and communicate with the 

 exterior through an oviduct on each side which opens laterally 

 near the posterior extremity of the trunk segment. The testes 

 are contained in the coelomic cavities of the tail segment. Sper- 

 matozoa, liberated directly into the cavity, are carried out 

 through a delicate sperm duct which is frequently dilated near 

 its extremity to form a seminal vesicle. 



In development, the fertilized egg undergoes cleavage to form 

 a typical gastrula. Two lateral folds of the entoderm extend 



Fig. 74. — Head of Sagitta showing bristle 

 like jaws extended. [Orig.). 



