DEVELOPMENT OF HYDROIDS. 59 
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 communi- 
cate with a canal following the margin 
of the disk. This is 
the water-vasculur sys- 
tem, communicating 
directly with the gas- 
a . tro-vascular eavity, or 
Coriemiretitie vthabid stomach. Four oan 
below a, and medusa-bud 
{gonophore) ata. Muchen- Cles hang from the 
larged.—After Agassiz. 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 u 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 Melicertwm, a Fig. 40.—Free Medu 
genus allied to Campanuluria, and a type °F Orme. 
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- 
