HYDROID POLYPS 473 



and hence possessed of a hydroid shape (nutritive zooids), others 

 are budded forth which are solely restricted to the task of reproducing 

 and spreading the species (generative zooids, or gonophores). These 

 gradually assume more and more the medusoid shape, and, finally 

 separating from the parent stock, swim about freely as true medusae, or 

 jelly-fish. These minute jelly-fish, however, are distinguished from the 

 Discophora, or acraspedote medusae, among other points, by the margin 

 of their umbrella never being lobed, but being, on the other hand, 

 provided with a border, or velum, passing inwards from the edge of the 

 umbrella, and thereby narrowing the aperture of the latter (they are 

 hence known as craspedote, i.e., fringed or veiled, medusa). From the 

 eggs of these medusae are again produced polyp colonies, which in their 

 turn produce the new generation of medusae by a process of budding, or 

 gemmation. Hence here, too, reproduction takes place by alternation of 

 generations. 



In several forms the generative zooids, or gonophores, do not 

 separate from the common stock; while, finally, there are species in 

 which the medusae produce medusse directly without the interposition 

 of an asexual generation (for example the Trachomedusm) . Thus, in 

 the former the medusoid, and in the latter the hydroid, generation 

 is omitted. 



CLASS II. : CORALS AND SEA-ANEMONES (ANTHOZOA)'. 



Polyp-shaped animals with an oesophageal tube, and with the body 

 cavity divided by vertical partitions, or " mesenteries." 



Anyone who has stood in front of a sea- water aquarium in which 

 naked pieces of rock are converted by Sea-Anemones {Actiniaria) into 

 flower-beds will acknowledge that these animals do not bear their name 

 without reason. These delicate, richly-coloured creatures, with the 

 numerous tentacles surrounding their mouth, indeed resemble animated 

 flowers (Anthozoa = flower animals), and convey some notion of the fairy- 

 like splendour displayed in the " coral gardens " of the Southern seas. 



With one species we have already become acquainted as a " com- 

 mensal " of the hermit crab (see p. 420). Like all the other forms, it 

 resembles a hydroid polyp in external appearance, but is distinguished 

 from it by the fact that the mouth is invaginated into the body cavity as 

 a so-called oesophageal tube, whilst the body cavity is divided, after the 

 manner of a poppy capsule, into a number of radiating compartments by 

 radial partitions, or mesenteries. These partitions are given off from the 

 internal body wall, and above are connected with the oesophageal tube, 



