XVII] 



INTERNAL ANATOMY 



383 



of modified ciliated epithelium. Clumps of isolated tactile cells 

 with long hairs surrounded by supporting cells are scattered over 

 the body and fins (Fig. 181). 



The alimentary canal is simple and straight. The mouth 

 with one exception is ventral and it leads into a pharynx which 

 traverses the head ; this passes into an intestine lined by a single 

 layer of ciliated epithelium amongst which are some glandular 

 cells. The anus is situated at the junction of the trunk and the 

 tail and is ventral. 



In Spadella marioni there is a glandular structure in the head 

 which may be connected with the excretion of waste nitrogenous 



FIG. 181. A. Transverse section through a Spadella cephaloptera in the region 

 of the ventral ganglion x about 200. B. Transverse section through a 



Sagitta bipunctata in the region of the ovary x about 120. 



Intestine. &. in A. Ventral ganglion. 6. in B. Ovary. c. in A. A 

 ciliated sense-organ. c. in B. Base of the left fin which has been cut off. 

 d. in A. Adhesive cells. d. in B. Left ventral muscle. e. in A. Ecto- 

 derm, e. in B. Ventral mesentery. /. Dorsal mesentery, g. Right fin. 



material, but no other excretory organ is known and no special re- 

 spiratory or circulatory organs exist. 



The Chaetognatha are hermaphrodite. The paired ovaries 

 (Figs. 180 and 181) lie in the trunk region of the body-cavity 

 supported by a lateral mesentery. When mature they almost fill 

 the cavity. The oviduct traverses the ovary. When the ova are 

 ripe they burst their way into the oviduct, and are fertilised in it, 

 for spermatozoa have been found in the oviduct. The oviducts 

 open to the exterior at the upper surface of the lateral fin, just 

 where the trunk passes into the tail. 



The median mesentery of the trunk region is continued through 

 the tail, dividing its cavity into two ; and in each of these lateral 



