172 



PRINCIPLES OF ANIMAL BIOLOGY 



freezing with impunity, but complete drying for a few days usually kills 

 them. When they germinate, the two valves are forced apart but may 

 remain attached to the growing animal for a long time. 



External Budding. — In external budding, the body wall is pushed 

 out at some point and develops the characteristic features of the animal. 



Fig. 142. — The hydroid, Bougainvillea ramosa, portion of a colony at left; medusa at right: 



mb, medusa bud; p, polyp. {After Allman.) 



In some species the bud is eventually pinched off, as it is in Hydra (Fig. 

 58, page 71). This is doubtless the original method. In other species 

 the buds remain attached, and colonies are produced, as is common in the 

 hydralike animals called hydroids. A typical one of these, Bougainvillea 

 ramosa (Fig. 142), forms a colony with a much-branched coenosarc (inte- 

 rior cellular portion) bearing at the ends of the branches flowerlike 



Fig. 143. — Diagram of structure of polyp (left) and medusa, with imaginary intermediate 

 form between; the plan of structure is the same. 



zooids, called polyps or hydraiilhs. Each polyp is provided with a 

 hypostomc, a conical projection at the distal end, around which is a 

 circlet of lentades. The coenosarc is surrounded b> the pcrisarc, a tough, 

 lifeless cuticle secreted by the cells of the coenosarc. The colony arises 

 from a branched rootlike structure, the hydrorhiza, which is attached 

 to a solid body such as a rock or log. This colony is produced by budding 

 without a separation of the l)uds from the parent. From the stalks 

 of many of the polyps, medusae (jellyfishes) are formed by budding. 



