190 '■"^ ANIMAL KINGDOM 



titioned gut. Medusa buds arise Irom the side of the polyp. The class 

 Scyphozoa includes most ol the larger jellyfish. The scyphozoan medusa 

 lacks a velum, the stomach cavity ol the polyp is subdivided by four 

 longitudinal partitions, and medusae are formed by transformation of 

 the end of the polyp so that the polyp mouth becomes the medusa 

 mouth. The class Anthozoa includes sea anemones and corals. The 

 polyps have a stomach cavity subdivided by 6, 8 or more partitions and 

 become sexually mature without transformation into a free-swimming 

 stage. Medusae are lacking. 



66. Class Hydrozoa 



The typical hydrozoan life history includes a juvenile polyp stage 

 that reproduces asexually and an adult medusa stage that reproduces 

 sexually. A full range of variations occurs, however, from species that 

 lack medusae to species that lack polyps. Hydrozoans lacking polyps 

 live in the open ocean where an attached stage is impractical; the 

 planula develops directly into a medusa. Polyps that lack medusae live 

 near the marine shores or in fresh water. The gonads develop on the 

 sides of the polyps, and a whole series of forms with various degrees of 

 suppression of the medusa stage indicates that these gonads represent 

 the last vestige of the medusa, appearing where medusa buds would 

 otherwise develop. 



Commonly the polyp is larger and longer-lived than the medusa. In 

 many hydrozoans most of the asexual buds of the polyp remain attached 

 to the parent to produce a colony of many polyps. The few buds that 

 creep off as frustules establish new colonies. Division of labor is frequent 

 in the colonial forms. Some polyps catch and eat food while others are 

 specialized for the production of medusae (Fig. 10.8). In a few species 

 additional polyps are modified into long clubs covered with nematocysts 

 which serve to protect the colony. 



The genus Obelia (Fig. 10.8) is representative of hydrozoans with 

 colonial polyps. The branching stalk and terminal polyps are covered 

 with a delicate horny sheath, the perisarc, secreted by the epidermis. It 

 is annulated in many places to provide flexibility as well as support for 

 the colony. The feeding polyps are typical. Polyps that produce medusae 

 have neither mouth nor tentacles ancl develop many medusa buds along 

 their sides. The medusae are about the size of polyps and do not grow 

 after they become free-swimming. In related genera the medusae never 

 become free of the polyp, but mature sexually and shed their gametes 

 while still attached. 



In the order Siphonophora of the class Hydrozoa the organisms are 

 remarkably complex. The planula does not become attached, but de- 

 velops into a polyp while swimming. The basal end of the polyp com- 

 monly develops an air sac to serve as a float. From this polyp a complex 

 colony of polyps and medusae develops by budding. Certain medusae 

 become permanent air floats while others are specialized for swimming. 

 Some of the polyps have no mouths, but are equipped with very long 

 tentacles covered with powerful nematocysts. Other polyps have mouths 



