16 STRUCTURE OF THE VERTEBRATES 



myosepta. These structures are important, for the muscle fibers 

 are attached to them, and they bind the myotomes to each 

 other and to the notochord. If the animal is thoroughly adult 

 the metameric gonads, or reproductive organs, will be visible. 

 The sexes cannot be distinguished except microscopically. In 

 sections the female ovary shows the spherical eggs or ova, and 

 the male testis has very minute sperms. 



Digestive Tract and Accessory Structures. Beginning at the 

 anterior end, the first nutritive structures are the buccal cirri. 

 In the living animal these are constantly in motion, and in 

 addition to a sensory function, probably assist in carrying a 

 stream of water with suspended food particles into the funnel, 

 and thus into the mouth. Within the funnel are two structures 

 which are supposed to have a sensory function: (1) a ciliated 

 groove (Groove of Hatschek) extending the length of the fun- 

 nel, slightly to the right of the dorsal midline; and (2) the 

 "wheel organ", patches of thickened, ciliated epithelium sur- 

 rounding the mouth and extending anteriorly like spatulate 

 fingers. The latter probably assists in passing the water toward 

 the mouth and pharynx. 



The mouth is guarded by a velum, or curtain, hanging from 

 the dorsal wall of the rounded opening. The mouth opens 

 directly into the pharynx, the largest single organ of the entire 

 animal. As the water which is forced through the mouth be- 

 comes more or less still in the enlarged cavity, the food par- 

 ticles tend to drop to the ventral floor where they are caught 

 in a groove along the mid-ventral line, the endostyle. This 

 groove is lined with cilia and mucous secreting cells, so that 

 the food is caught in the mucus and passed anteriorly toward 

 the mouth. At the level of the mouth the endostyle splits into 

 two grooves, the peripharyngeal grooves, which pass dorsally 

 around the opening and unite in the dorsal-median line as the 

 epipharyngeal groove. In this groove the cilia beat backward, 

 carrying the mucus with its attached food particles to the open- 

 ing of the stomach-intestine, a straight tube in which digestion 

 and absorption take place. The intestine opens to the outside 

 at the anus. 



Immediately posterior to the pharynx there is a single ventral 



