144 PHYLUM CCELENTERATA 



the formation of the siphonoglyphs. The gullet leads into the 

 gastrovascular space, which is the general internal cavity. 



The most prominent structures in this cavity are the mesen- 

 teries, 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 inclose the siphonoglyphs 

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

 primary mesenteries are secondary, tertiary, and quartemary 

 pairs. The gastrovascular space is thus divided into a large 

 number of partially separated longitudinal chambers. 



Note carefully the structure of the free edges of the mesenteries 

 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 repro- 

 ductive organs, the testes and ovaries, are also located in the 

 mesenteries, lying alongside the mesenterial filaments. The sexes 

 are separate and similar in appearance. 



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 disk 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. 



Make a cross section through the gullet and study the arrange- 

 ment of the mesenteries, the relation of the primary mesenteries 

 to the gullet, and the longitudinal muscles. 



Exercise 3. Draw a diagram of the cross section, showing these 

 features. 



Make a cross section through the body beneath the gullet. 



Exercise 4. Draw a diagram of the cross section, showing the ar- 

 rangement of the mesenteries. 



