330 ECHINODERMATA [CH. 



The alimentary canal can most easily be examined by carefully 

 Alimentary removing all the upper parts of the five arms in one 

 canal. piece ; this is done by cutting along both sides of 



each arm, then raising the upper part of the animal and clipping 

 through the intestine near the anus. By this means the animal is 

 separated into ah upper and lower half and all the internal organs 

 are displayed in one piece or the other. The alimentary canal is 

 then seen to consist of several regions clearly distinguished from one 

 another. It begins with an exceedingly short gullet which passes 

 at the lips into the buccal membrane already mentioned: the gullet 

 widens out above into a very loose baggy stomach which is produced 

 into ten short pouches, two situated at the beginning of each arm. 

 Above the stomach and communicating with it by a wide aperture 

 lies a flattened pentagonal bag, called the pyloric sac, and from 

 each of the five angles of this sac there is a tube given off which 

 runs into each arm, where it is soon divided into two parallel sacs, 

 each produced into a multitude of little, short pouches. These sacs 

 are called the pyloric caeca: caecum, Latin "blind," being a con- 

 venient zoological term for a blind pouch. Each of the pyloric 

 caeca is tied to the upper side of the arm, by two bands of trans- 

 parent membrane called mesenteries. From the centre of the 

 pyloric sac a short straight tube runs to the upper surface of the 

 animal where it opens by a minute anus: this tube is called the 

 rectum, a name, as we have seen, commonly given to the last 

 portion of the digestive tube. The rectum has attached to it two 

 branched tubes of a brown colour which open into it, called the 

 rectal glands. 



The reason of the division of the digestive sac into various 

 parts is of course the different uses to which they are put in the 

 life of the animal ; and we may stop for a moment to enquire what 

 these uses are. 



Star-fish feed chiefly on bivalve shell-fish, such as mussels, 

 cockles and clams, though they will attack almost any animal 

 which they can catch. Their mode of seizing their prey is very 

 curious. If they are attacking a bivalve, they bend all their five 

 arms down round it, thus arching up the central portion of the 

 body (Fig. 152). Then the stomach is pushed out, this being 

 rendered possible by the turning inside out of its edges, which, as 

 we saw above, are loose and baggy and wrapped around the fated 

 mollusc. The pushing out is effected by the contraction of some 

 muscle fibres in the body-wall: these tend to diminish the space 



