REPRODUCTION AND HEREDITY 261 



stages, transforms the Ephyra into the sexually mature Aurelia. 

 (See fig. 62.) 



Apart from terminal gemmation, the scyphostomae are able 

 like Hydra, to reproduce by lateral sprouting, but in that case 

 only polyps are produced. In many Medusae, as for instance the 

 brilliant Pelagia noctiluca, the original asexual polyp generation 

 is often entirely suppressed. In this case the ova produce at 

 once Epbyrae which change into Medusae. 



Somewhat different is the' cycle of development in the marine 

 types of polyps, the Hydrozoa. These animalcules usually form 

 small, but sometimes very extensive stocks which adhere to the 

 stones and shells by means of* many-branched roots, the Hydro- 

 rhiza. From this root rises the Hydrocaulus, the stem which 

 carries at the end of its branches the different citizens of its 

 colony, polyps and medusae. Hydrorhiza, as well as Hydrocaulus, 

 possess essentially the same structure as the polyps : they consist 

 of ectoderm and endoderm and an interposed supporting lamella. 

 As the stomach of the individuals is continued directly into 

 the stem and roots, all the food which is captured by any one 

 individual is distributed among the entire community. The 

 union of this little colony is so intimate that it has almost become 

 one individual of a higher order, and the single individuals 

 have sunk to the level of organs. In a still more distinct form this 

 is the case in the magnificent Siphonophora, which are nothing 

 but free-swimming polyp-stocks. Very few people would believe, 

 having fished one of these delicate structures from the sea, that 

 they hold in their hand not one organism, but a complete animal- 

 state. (See fig. 46.) 



These hydroid-stocks derive the requisite support from a 

 horn-like cover, a secretion of the outer cell-layer, which surrounds 

 the entire stock. In some species this armour ends at the base 

 of the polyps, but in others it widens here into a bell of con- 

 siderable size, into which the polyp withdraws, like a snail into 

 its shell, at the least sign of danger. 



As long as the little animal-stock is growing, new buds con- 

 tinue to sprout laterally from stem and branches, and develop 

 either into sexual polyps or into medusae. The polyps remain 

 throughout their lives fixed to the stem, but the adult medusae 

 separate from the stem to carry their sex-products through the 



