38 MORPHOLOGY OF INVERTEBRATE TYPES 



radial fibres; the expansion is due to the elasticity of the meso- 

 gloea. There is no velum in Aurelia, as is true for the entire Class 

 of Scyphozoa which therefore are called Acraspedce. What is 

 known as velarium in some Scyphomedusae is not a fold of the 

 ectoderm but a fold of the subumbrella containing endodermal 

 canals. There is no velarium in Aurelia and the application of 

 this name to the border of the disc between the base of the 

 tentacles and the margin is not correct. 



Mesogloea. The bulk of the medusa is formed by a 

 gelatinous, elastic mesoglcea. The mesogloea is not structure- 

 less as in Hydromedusae, but contains stellate and bipolar 

 cells. 



Gastro-vascular system. We have seen that the coelen- 

 teron of the Hydromedusae is already considerably more com- 

 plicated than that of the Hydropolyps. The ccelenteron of the 

 Scyphozoa is still more complicated and appears in the shape of a 

 highly differentiated gastro-vascular system. The mouth is 

 situated at the end of a short manubrium, and has more or less 

 the shape of a square. The angles of the opening are perradial in 

 position. The edge of the manubrium is drawn out to a con- 

 siderable length and forms four mouth arms which are also 

 perradial. Each of the four angles of the mouth continues as a 

 longitudinal groove to the end of the mouth-arm. Both edges of 

 the groove are fringed with a row of minute labial tentacles. 

 When both edges are in close contact the mouth appears not as a 

 square but as a cross or as a longitudinal fissure. It may be 

 added that the mouth-arms of the female are stouter than those 

 of the male. 



The mouth leads into the central cavity or stomach. The 

 stomach is produced into four large interradial gastric pouches, 

 between which the four perradial canals are situated. The 

 openings leading from the stomach into the pouches are called 

 the gastric ostia. On the floor of each gastric pouch is a groove 

 formed by two folds of the lining. These gonadial grooves extend 

 from the ostia to the middle of the pouches where they become 



