DEVELOPMENT OF HYDR0ID8. 



59 



Fig. 39. — Polypite of 

 Coryne mirabUis, vnth& bud 

 below a, and medusa-bud 

 (gouophore) at a. Much en- 

 larged. — After Agassiz. 



living Millepora, unless handled with great care, severely 

 stings the hand of the collector. 

 We now come to Hydroids which throw off a free naked- 

 eyed medusa from the hydrarium (Fig. 

 39). From the centre of these free 

 bell-shaped, minute jelly-fishes depends 

 a hollow, open sac called the manu- 

 brium, the cavity of which (stomach) 

 opens into usually four canals, which 

 radiate from the hollow or stomach in 

 the centre of the disk and communis 

 cate with a canal following the margin 

 of the disk. This is 

 the water-vascular sys- 

 tem, communicating 

 directly with the gas- 

 tro-vascular cavity, or 

 stomach. Four tenta- 

 cles hang from the 

 disk, and simple eye- 

 spots and otolithic sacs (simple ears) 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 walls of the ovary. 



The young is at first oval, ciliated all 

 over the surface of the body, and is called a 

 planula. The planula, as in Melicertuvi, a Kg. 40.— Free Medu- 

 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 itself 

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



