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 esophagus, 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 cecum. The intestine, together with its 
cecum, 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 
