DEVELOPMENT OF HYDROIDti. 



59 



:-.'.!. Polypite of 



'Corynetfiirobilis, \\ithabud stomach, 

 liolow a, and medusa-bud 

 (gouophoiv) at ft. Mucheii- 

 largcd. After Agassiz. 



living Millepora, unless handled with great care, severely 

 stings the hand of the collector. 



"\Ve now come to llydroids which throw off a free naked- 

 eyed medusa from the hydrarinm (Fig. 

 30). From the centre of these free 

 bell-shaped, minute jelly-fishes depends 

 a hollow, open sac called the manu- 

 briuin, the cavity of Avhich (stomach) 

 opens into usually four canals, which 

 radiate from the holloAv or stomach in 

 the centre of the disk and communi- 

 cate with a canal following the margin 

 of the disk. This is 

 the water-vascular sys- 

 tem, communicating 

 directly Avith the gas- 

 tro-vascular cavity, or 

 Four tenta- 

 cles hang from the 

 disk, and simple eye- 

 spots and otolithic sacs (simple cars) are usu- 

 ally present and situated at regular inter- 

 vals around the edge of the disk. Such is 

 the typical form of all the free-swimming 

 Hydroids. They are said, in a few cases, 

 to possess a well-developed continuous ner- 

 vous system, consisting of a nervous ring 

 around the disk (Romanes). They are bi- 

 sexual, the ovaries or spermaries being de- 

 veloped on the radiating canals, the embryo 

 escaping into the surrounding water by rup- 

 turing the Avails of the ovary. 



The young is at first oval, ciliated all 

 over the surface of the body, and is called a 

 plannla. The planula, as in Melicertwn, a Fig. 40. Free Me. 

 genus allied to Campanularia, and a type 

 of most marine Hydroids, at first spherical, becomes pear- 

 shaped, and after swimming about for a time attaches itseli 

 to some object. It then elongates, a horny sheath (peri- 



' 



