REPRODUCTION AND DEVELOPMENT 259 



an ovary or a spcrmary on the same individual, or by budding, the 

 bud becoming detached to form a separate individual. 



The Hydroids. The hydroids are mostly marine Hydrozoa 

 and exhibit metagenesis. The asexual form is attached to rocks or 

 other material and consists of a branching, diploblastic tube (Fig, 

 176), The walls of the tube in many varieties are covered with a 

 transparent non-cellular layer secreted by the ectodermal cells, 

 known as the perisarc. Some branches end in a structure much like 

 a single hydra, consisting of an expanded bell-shaped hydraxth 

 around the rim of which are tentacles armed with nematocysts. 

 The mouth is in a central eminence known as the hypostome. 

 This constitutes a feeding hydranth. On other branches are hy- 

 dranths, quite different in structure, which are specialized as re- 

 productive hydranths. The reproductive hydranth consists of a 

 central shaft, the goxostyle, on which develop a number of flat 

 plates of cells known as medusa buds. It is covered with a layer 

 known as the gonotheca, which is continuous with the perisarc 

 of the stem. At the extreme end of the reproductive hydranth 

 is a pore, the gonopore. When the medusa buds are mature 

 they separate from the gonostyle and escape through the gonopore 

 into the water. Here they develop into free swimming forms known 

 as MEDUSA. The processes of medusa bud formation and the de- 

 velopment of medusae are entirely asexual. 



The free swimming medusae are sexual animals (Fig. 176). Their 

 general shape is similar to that of an umbrella. The tentacles are 

 suspended from the rim. The mouth is at the end of a short tube 

 suspended in the cavity formed by the concave under surface of 

 the animal, similar in position to the handle of an umbrella. This 

 tube is continuous with four radial alimentary canals, the position 

 of which may be compared with that of the ribs of an umbrella. At 

 the rim these four radial canals enter a circular canal that traverses 

 the entire circumference of the animal. The sex organs develop on 

 the walls of the radial canals. The sexes are separate. An individual 



