102 Coelentera. 



On the other hand, reproduction is often fairly complex. 

 A metamorphosis is very general^ the animal leaving the egg as 

 a very simple organism, ciliated and free-swimming, but without 

 tentacles; whilst later, usually after it has become sessile, it is 

 modified to a greater or less extent. Moreover, asexual repro- 

 duction by fission or biidding often attains considerable importance ; 

 and colonies are frequently formed. In many species a regular 

 alternation of generations occurs. 



The Coelentera are almost all marine, only quite a few inhabit 

 fresh water. Their nettle cells enable them to prey upon even 

 relatively large and powerful animals, which are digested by the 

 endoderm cells of the gastric cavity, the indigestible portions being 

 thrown out through the mouth. 



Class 1. Hydrozoa. 



This group is characterised by the general occurrence of an 

 alternation of generations, and by the great dis- 

 similarity of the sexual and asexual generations. 



The asexual generation (the polypoid individuals), presents 

 the simplest coelenterate form, which consists of a simple sac with the 

 mouth at one end and the body-wall of the usual layers (c/. Fig. 58, A) ; 

 at the uj)per end of the animal there is a varying number of 

 tentacles, which are usually arranged some little distance from 

 the centre of the disc. The polyps are usually attached (often 

 immovably) by their free ends to some foreign object. Most of them 

 form colonies by budding. 



The sexual generation, the medusoid individuals, are 

 characterised by the broadening out of that portion of the body 

 which corresponds to the lower end of the polyp, to form a circular 

 convex disc, the umbrella (c/. Fig. ' 58, B) in which the middle 

 layer is specially well-developed on the convex side ; radial processes 

 of the alimentary canal extend into the disc as the radial canals, 

 the ends of which are generally united by a ring canal running 

 round the umbrella close to its edge. The medusa is typically free- 

 swimming (for exceptions see the HydromedusEe and Siphonophora) with 

 the disc turned upwards ; from the centre of the umbrella depends a 

 tubular portion corresponding to the upper extremity of the polyp, 

 and forming a longer or shorter manubrium perforated by the 

 mouth. From the edge of the bell hang contractile marginal tenta- 

 cles which are richly supplied with nettle cells ; simple auditory 

 and optic organs are present on the margin, and below the 

 epidermis (at any rate, in the Hydromedusae) , numerous nerve cells 

 are distributed all round the disc, forming a nerve ring with 

 their processes. On the concave lower side, there is a layer of 



