Glass 1. Hydrozoa. lOS 



circular muscle fibres^ often tranversely striped, which make 

 the umbrella extremely concave when they contract, and thus move 

 the whole animal. As a rule , the medusee are of separate 

 sexes; the formation of ova and spermatozoa will be considered 

 in the individual orders. 



A ciliated larva develops from the fertilised egg, fixes itself, and 

 develops tentacles; polyps which have originated in this way may 

 form colonies, but some remain solitary. The medusoid form arises 

 again as a bud from the polyp, or by its transverse fission. More 

 rarely the egg of the medusa develops direct into a new medusa,, 

 in which case the polypoid form (and with it, alternation of generations) 

 does not occur. 



In structure, the Hydrozoa generally display a markedly radial 

 symmetry ; the number of rays is usually four or some multiple of 

 four, more rarely six. 



Order 1. HydromedusaS. {Craspedota). 



The polyp generation consists of hydroids, which usually form 

 colonies, but are occasionally solitary. The tubular, often extremely 

 elongated, body of the polyp is almost always surrounded by a 

 cuticle, which forms a chitinous case, usually thin, more rarely 

 thick and calcified. This does not surround the whole body ; a 

 portion of the upper part remains uncovered. Sometimes the tube 

 has a cup-shaped expansion above, into which the naked, broader, 

 tentacle-bearing portion of the polyp can be withdrawn. The 

 tentacles are arranged in one or m.ore circles below the mouth, 

 or more irregularly at the upper end of the animal (Fig. 60.) 

 They are not generally hollow as in Corals, but possess a solid 

 axis formed of a single row of endoderm cells; outside this there 

 is a continuation of mesoglsea, and most externally ectoderm with 

 abundant nettle cells. The colonies formed by the polyps are 

 generally small compared with those of the corals (c/. below), of 

 which the solitary individuals are similarly very much larger. 

 Sometimes the colonies are formed on a definite system of branch- 

 ing : the polyp developed from the egg continually increases in 

 length, whilst at the same time it forms lateral buds, of which the 

 oldest are at the base; these develop into new individuals (lateral 

 polyps), which again, branch in the same way as the main stem. In 

 other cases the growth of the first polyp ceases; only one or two 

 lateral branches arise from it, each of which stops growing after 

 forming one or two buds (c/. the cymose branch-system of plants). 

 The lower end of the colony is provided with hollow, root-like 

 outgrowths of the body-wall, covered by the cuticle; by means 

 of these stolons, which are sometimes connected to form a 



