THYONE. 85 



jets of water ejected by the living specimen were thrown from 

 these tubes. Can you understand how they serve for respira- 

 tion? The walls of the tubes composing the trees are glandular 

 and may thus serve to excrete wastes. Notice the muscles that 

 radiate from the walls of the cloaca to the body-wall. What 

 is their function? 



Make a drawing of the cloaca and respiratory trees. 

 Reproductive System.— The single gonad (ovary or testis) 

 occupies a median dorsal position in the anterior part of the 

 body-cavity. It is composed of a multitude of filaments, which 

 join to make a brush. This brush projects backward into the 

 body-cavity. The duct of the organ lies along the dorsal mid- 

 line, betw^een the right and left dorsal muscle bands, and leads 

 to the opening upon the small papilla near the mouth that has 

 already been noticed. 



Water-vascular System.— 1. The circular canal can be found 

 in favorable specimens, surrounding the deeper portions of the 

 esophagus. It gives rise to one or two Polian vesicles, which 

 are very large and hang down into the body-cavity. 



2. The five radial canals (homologous with the radial canals 

 of the starfish and sea-urchin) originate from the water-ring, 

 pass forward and then backward, and end near the cloaca. 

 The radial canals take the general course of the longitudinal 

 muscle bands and He between this muscle band and the body- 

 wall. The radial canal may be seen if the muscle band is care- 

 fully removed. 



3. Ten forwardly directed canals, the tentacular canals ^ 

 leave the radial canals near the water-ring and pass into the 

 tentacles, which may be homologized with tube feet. 



4. The stone canal and madreporite are much reduced in 

 holothurians. The madreporite, except in larvse and very 

 young specimens, is not found on the outer surface. The 

 stone canal leads obliquely backward from the water-ring, 

 toward the dorsal body- wall, to join a small calcareous body, the 

 madreporite, which lies in the body-cavity and is not perfor- 

 ated. Does this give you a reason for the presence of large 



