36 Invertebrate Zoology. 



A HOLOTHURIAN 

 (Thy one briareus). 



NOTE the general shape of the body, the point of origin 

 of the tentacles, their number, and the oral and anal open- 

 ings. The body is covered with papilliform ambulacral 

 suckers, which, by careful observation, will be found to be 

 arranged in five broad, meridionally disposed bands. On the 

 upper side of the animal the suckers are less abundant than 

 on the lower side. In the angle between the upper ten- 

 tacles, the opening from the genital duct will be found. 

 Examine the oral opening. Are there teeth? Are the ten 

 large circum-oral ambulacral suckers of the sea-urchin rep- 

 resented? 



Make a drawing of the animal as seen from the side. 



With a pair of scissors open the animal longitudinally 

 along the middle of the lower surface. Why is it not strictly 

 correct to here speak of the " ventral surface " ? 



The Digestive System is greatly elongated and extends 

 as a coiled tube which partly nils the body-cavity. It is poorly 

 suspended to the body-walls by a delicate " mesentery," 

 and in certain portions of its course becomes somewhat 

 glandular. The mouth opens into an oesophagus which is 

 surrounded by a cartilaginous structure that recalls the dentary 

 apparatus of the urchin. The oesophagus soon enlarges into 



