228 Elementary Biology. 



The edge of the bell has an inwardly directed rim, or velum, 

 and carries four or a multiple of four long tentacles (fig. 1 1 7). 

 The tongue, or manubrium, is not unlike the head of an 

 alimentary zooid turned upside, down and denuded of 

 tentacles. The cavity of the manubrium leads into a quad- 

 rangular space, or enteron, which again is prolonged into 

 four radial canals, hollowed out of the tissue of the disc 

 and united by a circular canal which runs round the rim. 

 Prolongations of this canal occupy the axes of the ten- 

 tacles. The cavity of the manubrium, enteric cavity, and 



FIG. 118. MUSCLE-FIBRES AND NERVE CELLS FROM THE NECTO-CALYX 

 OK Aurelia aurita. (Schafer.) 



the entire series of canals are lined by endoderm. In fact, 

 the medusoid is merely a greatly modified zooid (which has 

 arisen as a bud from the reproductive zooid) turned upside 

 down and free swimming. The medusoid, however, is a 

 much more highly organised creature than its parent. In 

 the first place, its tissues are more highly differentiated. In 

 the hydroid form there were no cells specially set apart to 

 contract ; each cell retained that power. In the gonophore 

 there are often distinct muscular fibres scattered through 

 the nectocalyx, by the rhythmic contraction of which the 



