22 BRANCH CCELENTERATA 



and connected, the whole colony shares the food thus supplied. 

 When disturbed the polyp withdraws into the hydrotheca for 

 protection. 



Blastostyles. — But while the majority of the members of this 

 colony are hydra-like, tentacle-bearing polyps which reproduce 

 by budding only, and can enlarge the original colony, they 

 have no power of directly producing a new colony in a more 

 favorable position. There is, therefore, another set of individ- 

 uals (see Fig. 11). These, while forming a part of this tubular 

 colony, are modified in their form for a particular function. 

 They are situated toward the proximal region of the colony and 

 are long, cylindric bodies, known as blastostyles, each of which is 

 enclosed in a transparent case, the gonotheca. These are the 

 reproductive zooids, and bear small lateral circular buds called 

 medusa buds, which, as they mature, become detached and pass 

 out through an opening now formed at the end of the gonotheca. 



Alternation of Generations. — These medusa buds are sexual 

 and dioecious, i. e., the sexes are separate, one individual producing 

 the ova and another the sperm cells. After fertilization, which 

 takes place in the water, the egg develops into a simple, free- 

 swimming ciliated larva, the planula, which soon attaches itself 

 to some object, develops into a polyp, and, by budding, forms 

 a new colony. This regular reproduction by budding, and then 

 by eggs, and then by budding again is called alternation of genera- 

 tions, or metagenesis. 



Medusae. — Careful study shows that the Medusa is only a 

 highly developed or modified zooid. The cylindric body has 

 been developed into a disk or umbrella-shaped body (Fig. 14) ; 

 the long axis has been greatly shortened and is suspended be- 

 neath the center of the sub-um])rella, as the under surface of 

 the disk is called, where it takes the name of mariubrium, or 

 " handle." At the free end of this manubrium is the mouth, 

 which opens into the gastric cavity that occupies the whole 

 interior of the handle. 



At the base of the manubrium four radial canals, equally 

 distant from each other, are sent out to the circular canal, 

 which runs around the margin of the umbrella, but within its 

 substance. Thus, the food taken into the mouth is distributed 

 to the whole animal. The whole canal system is lined by endo- 



