CCELENTERATA 



179 



may communicate with a circular canal which passes around the 

 bell near the bases of the tentacles. This whole internal cavity 

 is lined with entoderm, and therefore no portion of it represents 

 a ccelom, but is merely a much-modified digestive tract (Fig. 

 82, 6). 



FIG. 82. 



-VH CC ' T" * .-^ 



PlG. 82. Sections of types of Ccelenterates (diagrammatic): T (longitudinal) and 2 (transverse) 

 of a tubular hydroid; 3, Sea Anemone (longitudinal) ; 4, same (transverse, at the level of the upper 

 dotted line); 5, same (transverse, at the level of the lower dotted line); 6, longitudinal or vertica 

 section of a Medusa; 7, transverse section of same at the level of the dotted line. The continuous 

 line is ectoderm, the broken line, entoderm, and the stippled portion, mesenchyma. c.c., circular 

 canal; g, gullet; g.p., gastro- vascular cavity; m, mouth; ma., manubrium; mes., mesentery; mes. 1 

 directive mesentery; o, ostium; r. c., radial canal; t, tentacle; v, velum. 



Questions on the figures. By a careful comparison of the diagrams what 

 points of similarity do you find in these three types ? What are the principal points 

 of difference? Examine similar diagrams in other texts. Why is Ccdenterate an 

 appropriate name for all. 



The bell is comparable to an inverted polyp in which the 

 main axis has become much shortened, accompanied by a 

 thickening of the body in the direction of the other axes. 1 

 The gastro-vascular cavity is further modified by the increase 

 of the mesenchyma of the aboral disc and by a union of the 

 oral and aboral walls of the cavity in certain regions. The 



1 See Textbook of Zoology, Parker and Haswell, Vol. I, p. 127, Pig. 89. 



