48o CHARACTERS OF INVERTEBRATE ANIMALS 



the margin of the umbrella. The eggs give rise to hydra-like 

 individuals, each of which develops a fresh colony by means of 

 budding. 



The Hydrozoa may conveniently be divided into two orders: 

 I. Budding Hydroids (Hydromeduss) ; and 2. Splitting Hydroids 

 ( Scyphomedusae). 



Order i. — Budding Hydroids (Hydromedusae) 



Obelia is a good typical example of the order, but in by no 

 means all cases are egg-producing buds set free as medusae, 

 and a complete series of examples may be selected which range 

 from that condition down to the state of things found in Hydra, 

 where each egg is produced in a little swelling which has no 

 resemblance whatever to a medusa. The intermediate stages 

 are represented by cases where the egg-producing buds resemble 

 medusae but remain attached to the colony, and other cases where 

 such buds may be compared to medusae in which some of the 

 typical features have been, as it were, suppressed. 



Obelia is a type of one large division in which the fixed 

 stage is distinguished by the possession of cups into which the 

 individuals can be withdrawn, while if free-swimming jelly-fish 

 are produced their sense organs are usually auditory vesicles. 

 Other common British genera are Sertularia and Phtmularia. 



In another large group the investing skeleton ends abruptly 

 at the base of each polype and does not expand into a cup, while 

 the medusae, if liberated as free-swimming individuals, usually 

 possess eye -spots instead of auditory vesicles round the margin 

 of the umbrella. Tubularia is a typical British genus, in which 

 the polypes are comparatively large, and free-swimming medusae 

 are not developed. It is also customary to place Hydra in this 

 group, as well as a very interesting marine genus, Protokydra, of 

 somewhat similar character, though it has no tentacles, and is to 

 be regarded as the simplest known member of the Hydrozoa. 



Although most corals belong to the Anthozoa, there are a 

 few cases of species belonging to the Hydrozoa which, instead 

 of secreting a horny investment, develop a firm calcareous 

 skeleton, and superficially resemble the true corals, though iu 

 reality sharply marked off from them by the structure of the 

 soft parts. Representative genera are Millepora and Stylaster, 



