ii6 PHYLUM ECHINODERMATA 



arms about the peristome ; note also the interradial partitions 

 which separate the base of the arms. 



Exercise 5. Make a hfe-size drawing showing these features. 



Internal Anatomy. Remove the entire aboral body wall from 

 the trivium and the central disk of the fresh or preserved speci- 

 men, with the exception of the madreporite which must not be 

 removed, being very careful not to injure the organs beneath. 

 Study the internal organs and observe the following systems : 



The Digestive System. Observe the large sacHke stomach, 

 which almost fills the central disk. Its walls are much folded, 

 and five short, baglike pouches extend from it into the five arms. 

 When the animal feeds the stomach is everted and thrust out 

 through the mouth and about its prey. It is drawn in again by 

 means of five pairs of retractor muscles, which connect the stom- 

 ach pouches with the inner surface of the ambulacral grooves. 

 Find the pair of retractors belonging to each stomach pouch. 

 Communicating with the aboral portion of the stomach are five 

 large, radial digestive glands, which are usually called livers. Each 

 gland almost fills an arm ; it is made up of two main trunks, from 

 which project numerous side branches ; the two ducts leading 

 from the two trunks in each arm unite to form a single duct which 

 passes to the stomach. Each trunk is suspended from the aboral 

 wall of the arm by two mesenteries. Find the mesenteries in one 

 of the bivial arms. Study the structure of the livers. The stomach 

 is connected with the mouth by a short oesophagus, and from its 

 upper surface a short, slender intestine passes to the anus. Con- 

 nected with the intestine is a small branched diverticulum, the 

 intestinal caecum. The intestine, together with its caecum, may 

 have been removed when the aboral body wall was taken off. If 

 this is the case look for them on the portion of the aboral wall 

 which was taken off and notice also the position of the anus. 



The Reproductive System. The sexes of the starfishes are sep- 

 arate. The sexual organs are branched glandular organs, ten in 

 number, which lie in the proximal portion of the rays and open 

 to the outside through minute pores in the aboral walls of the 

 interrays. Two glands will be found in each ray extending from 



