IV 



PHYLUM CXELENTERATA 



139 



or per-radii. Halfway bet ween any two per-radii a radius of the second order, 

 or inter-radius, may be taken ; halfway between any per-radius and the inter- 

 radius on either side a radius of the third order, or ad-radius, and halfway 

 between any ad-radius and the adjacent per- or inter-radius, a radius of the 

 fourih ordc.r, or su'i-radiits. Thus there are four per-radii, four inter-radii, 

 eight ad-radii, and sixteen sub -radii. In Obelia the radial canals, the angles of 

 the mouth, and four of the tentacles are per-radial, four more tentacles are 

 inter-radial, and the remaining eight tentacles, bearing the lithocysts, are 

 ad-radial. The sub-radii are of no importance in this particular form. 



FIG. 103. Projections of polype (A) and medusa (JB), showing the various orders of radii 

 gon. gonad ; mnb. manubriuni. 



Reproduction. In the description of the fixed Obelia-colony 

 no mention was made of cells set apart for reproduction, like 

 the ova and sperms of a sponge. As a matter of fact, such sexual 



