88 CCELENTERATA 



tain thousands of individuals, and grow from one another by process of 

 budding. 



The medusa type is seen in its simplest form in the usually minute 

 hydromedusans and in a more complex form in the larger scyphomedu- 

 sans. In all these animals the body is more or less bell or disc-shaped, 

 the convex side, which is called the exumbrella, corresponding to the 

 attached end of the hydroid polyp, while from the center of the concave 

 side, which is called the subumbrella, extends the manubrium, a more or 

 less cylindrical but often branched projection, at the end of which is 

 the mouth. Tentacles may be present on the manubrium, at the edge 

 of the bell, on the subumbrella or the exumbrella, and may be long or 

 short and flexible or rigid. 



Two distinct types of medusae are met with. Those of one type are 

 called craspedote medusae (Fig. 147, B), because they possess a velum (7). 

 This organ is a ridge or membrane containing epithelial muscle fibres, 

 which extends inwards toward the manubrium from the entire edge of 

 the subumbrella. These medusae are almost all small, being usually less 

 than 2 cm. in diameter, although some are larger, JEquorea tenuis attain- 

 ing a diameter of 10 cm. and 2Equorea forskalea of the Mediterranean 

 one of 40 cm. and, excepting the Narcomedusae, have a plain, unscalloped 

 edge. Those of the other type are called acraspedote medusae (Fig. 216) ; 

 these lack the velum or have it in a rudimentary form and possess a 

 scalloped outer edge, as well as other special features; they are also usu- 

 ally large, some having a diameter of a meter or more. A certain number, 

 however, are small, with a diameter of less than a centimeter. 



In Hydra and the Anthozoa the hydroid or pol3q3 type of structure 

 alone prevails, and the animals produce, either by budding or by sexual 

 methods, young individuals which develop directly into adults similar to 

 the parents. In most Trachomedusae and Narcomedusae, so far as known, 

 the medusa type alone prevails, the young developing directly into free- 

 swimming medusae. In the Ilydromedusae and Scypliomedusae, on the 

 other hand, both types may prevail in the same species, and the phenome- 

 non of the alternation of generations is exhibited, an asexual generation, 

 which is the hj^droid, producing by budding a sexual generation, which 

 in these animals is the medusoid generation. The medusoids are either 

 male or female and produce embiyos called planulae, which after a period 

 of free life attach themselves to some fixed object and become hydroid 

 polyps, the medusoid buds, in certain cases, remaining attached to the 

 parent hydroid, and in others becoming free-swimming jellyfish. 



In all the Cnidaria, the body wall consists of the outer ectoderm, the 

 inner entoderm and the middle mesoglea (Fig. 131). The ectoderm consists 

 of a single layer of cells, among the inner ends of which are small inter- 



