150 



METAZOAN PHYLA 



fixed, develops into an individual somewhat like a hydra, known as a 

 scyphistoma. This forms at its outer end a series of saucer-hke buds 

 piled one upon another, which as they grow older gradually develop into 

 medusae ; each of these buds is called an ephyra. When the scyphystoma 

 has developed a whole series of such buds it is called a strobila. As the 

 ephyrae are formed successively from the outer end of the parent indi- 

 vidual it follows that the oldest will always be at the free end of the pile 

 and the youngest at the lower end next to the parent. The ephyrae, 

 when freed, gradually develop into mature jellyfishes. 



Sperm 

 Gonads cell from 

 anofher 



Fig. 66. — Diagram illustrating metagenesis, and also polymorphism, in the life history 

 of a species of Obelia. A, portion of a colony, with hydranths and a gonangium; these and 

 the medusa show three forms of the same species, which is polymorphism. B, sexual 

 medusa, produced in the gonangium by budding and set free in the water. An egg cell 

 from this is fertilized by a sperm cell from another animal, passes through two-cell and 

 four-cell stages, and in time becomes a blastula, C. This passes through a gastrula stage 

 and finally becomes a ciliated planula larva, D. The larva settles down, becomes attached 

 (E), and from it a new colony is formed (F and A). 



179. Metagenesis. — The phenomenon of a budding generation being 

 followed by a generation which produces egg cells and sperm cells is 

 known as alternation of generations, or metagenesis. The marine 

 hydroids very generally illustrate this phenomenon, and Obelia may be 

 taken as an example (Fig. 66). A colony of Obelia consists of individuals 

 that have been asexually produced by budding from a parent which 

 in turn was developed from a sexually produced planula. These polyps 

 are of two types — nutritive and reproductive. The nutritive individuals, 

 or hydranfhsy provide food both for themselves and for the reproductive 



