METRIDIUM 179 



Exercise 1. Draw the expanded animal, showing the column and 

 the disc, with the mouth and the tentacles. 



Internal anatomy. Cut the animal into halves by a longitudinal 

 incision passing at right angles to the mouth and from the disc 

 to the foot. The mouth will be seen to open into a flattened 

 tube with more or less corrugated walls, called the gullet. Note 

 the formation of the siphonoglyphs. The gullet leads into the 

 gastro-vascular space, which is the general internal cavity of the 

 animal. 



The most prominent structures in this cavity are the mesenteries, 

 which are longitudinal partitions extending from the outer wall 

 of the body inward toward its center. These mesenteries will 

 be seen to occur in pairs ; six of these pairs, called the primary 

 mesenteries, join the body-wall with the wall of the gullet. The 

 pair at each angle of the gullet which enclose the siphonoglyphs 

 between them are called the directives. Between the six pairs of 

 primary mesenteries are secondary, tertiary, and quarternary pairs. 

 The gastro-vascular space is thus divided into a large number 

 of partially separated longitudinal chambers. 



Note carefully the structure of the free edges of the mesen- 

 teries below the gullet. The thickened corrugated structure 

 which forms the edge is the mesenterial filament ; it contains 

 digestive glands. From the base of the mesentery extend the 

 acontia. The reproductive organs, the testes and ovaries, are 

 also located in the mesenteries, lying alongside the mesenterial 

 filaments. 



Note carefully the position of the longitudinal muscle bands, 

 one of which is present on the surface of each mesentery. It is 

 by means of these muscles that the body is contracted. A circu- 

 lar muscle in the disc closes the mouth by its contraction and aids 

 in drawing in the tentacles. 



Exercise 2. Draw a semidiagrammatic view of the cut surface of 

 the animal, showing these features. 



