A STARFISH 145 



Internal anatomy. Remove the entire aboral body-wall from 

 the trivium and the central disc of the fresh or preserved 

 specimen, with the exception of the madreporic plate 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 sac-like stomach, 

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

 and five short, bag-like 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 con- 

 nect the stomach pouches with the inner surface of the ambu- 

 lacral 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. Connected with the intestine is a small branched diver- 

 ticulum, the intestinal caecum. The intestine, together with its 

 caecum, may have been removed when the aboral body-wall was 

 taken off. If this be 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 sepa- 

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

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



