382 



CHAETOGNATHA 



[CH. 



M-b 



FIG. 180. A ventral view 

 of Sagitta hexaptera x 3. 

 From 0. Hertweg. 



a. Mouth. &. Intestine, 

 c. Anus. d. Ventral 

 ganglion. e. Movable 

 bristles on the head. 

 /. Spines on the head. 

 g. Ovary, h. Oviduct. 

 i. Vas deferens. j. Tes- 

 tis. k. Vesicula semi- 

 nalis. 



others project from the surface of the 

 body and are known as adhesive cells 

 (Fig. 181). Beneath the epithelium is 

 a thin layer of jelly called the base- 

 ment membrane, and beneath this a 

 layer of muscles. Anteriorly the muscles 

 are broken up into numerous bundles 

 which fill the cavity of the head, but in 

 the trunk and tail the muscles form four 

 distinct bundles, bilaterally arranged, two 

 dorsal and two ventral. 



There are no circular muscles either 

 in the skin outside the coelom or sur- 

 rounding the gut. Hence no peristalsis 

 can take place, and this is the reason 

 that the intestine is ciliated, for the 

 action of the cilia is necessary to propel 

 the food through it. Arrow-worms pro- 

 gress hy alternate bendings of the body 

 first to one side and then to the other very 

 much as fish do, the stiff cuticle acting 

 the same part in the Arrow-worm as the 

 back-bone in a fish, i.e. of enabling the 

 body to spring back into its original 

 form, when the contraction of the muscle 

 ceases. 



The nervous system consists of a 

 dorsally placed ganglion in the head 

 which gives off two lateral nerves ; these 

 pass round the alimentary canal and end 

 in a ventral ganglion situated (Figs. 180 

 and 181) near the centre of the body. 

 The cerebral ganglion gives off nerves to 

 the eyes, the olfactory organ, muscles, 

 etc., and both it and the ventral ganglion 

 are connected with a tangle of nerve- 

 fibrils lying at the base of the ectoderm. 

 A pair of eyes exist on the upper part 

 of the head, and behind the eyes an 

 organ to which an olfactory function has 

 been assigned. This consists of a ring 



