HYDROIDS. 



81 



equidistant points stretch away to the rim of the bell, where they are all connected by 

 a tube passing around the rim. By means of these gastro vascular canals nutritive mat- 

 ter from the stomach is carried all over the body. Stretching partly across the open- 

 ing into the bell, is a thin, centrally-perforated membrane called the velum or veil. 

 After one of these niedusse has been completely developed on the proboscis of the 

 hydroid of Pennaria, it is freed from the proboscis by a constriction which cuts in two 

 the small peduncle by which it had been attached, and the medusa floats away free in the 

 water. It is not left to the mercy of currents, however, but is provided with a rather 

 peculiar locomotor apparatus. The cavity of the bell being filled with water, its mus- 

 cular walls are powerfully contracted, and the water being ejected from the opening 

 in the velum, the medusa is forced through the water in the opposite direction ; then 

 expanding its bell by other muscles, it is ready to contract again and send itself still 

 farther on its way. The tentacles and outer surface of the medusa are well supplied 

 with cnidocells with which they defend themselves and kill their prey in the same 

 manner as Hydra, and as the zooids in the hydroid colony. The medusse are sexual 

 zooids, the' sexes being separate, and in the case of Pennaria, the male and female 

 elements are developed within the walls of the proboscis. From a fertilized egg a 

 planula is developed, which in turn gives rise to a hydroid colony of the Pennaria 

 kind. The life-cycle is thus more complicated than in Eudendrium by the introduc- 

 tion of the medusa stage. The length of an average Pennaria medusa is about one- 

 sixteenth of an inch. 



Objects of more exquisite beauty than some of these hydroid-medusse do not per- 

 exist. Each minute crystal chalice with its beautifully curved outline, elongated, 

 delicate tentacles gently coiling and uncoiling, and its slender proboscis which hangs 

 like a lamp in its centre, lighting it with a soft phosphorescent 

 glow as it swims with most perfect grace at the surface of the 

 ocean, is the very type of delicate beauty, suggesting the won- 

 ders of fairy-land. 



The dredge frequently brings itp delicate pink or flesh-colored 

 hydroids consisting of single stems, each supporting a single 

 hydranth. This hydranth bears two sets of arms, those around 

 the free end of the proboscis being much shorter than those nearer 

 the base. This form was called by Agassiz Corymorpha pendula. 

 It lives with the base imbedded in the mud, and grows to a 

 length of four inches. The investing envelope is very soft, and 

 the animal is able to greatly modify the shape of the stalk and pro- 

 boscis. The medusa buds never become free-swimming jelly-fishes, 

 while the hydroid stem always bears a single head or hydranth, a 

 fact which led AUman to refer it to the genus Monocaulis. 



The genus Tubularia and the closely allied Thamnocnidia, are represented on our 

 coasts by several species. The hydranths are borne on slender stems, and form col- 

 onies reaching sontetimes a height of eight or ten inches. Under a low power of the 

 microscope, the beauties of the animals stand revealed, far exceeding the power of any 

 pen to describe or brush to paint. The hydranth is surrounded with two circles of 

 tentacles, and from between the lower ones the reproductive zooids hang down like 

 bunches of grapes, or they cluster around the proboscis inside the outer circle of ten- 

 tacles, so that it requires no very vivid imagination to imagine the whole a delicate 

 fruit-dish filled with the most beautiful fruit. From these raceme-like clusters the 

 -6 



Fig. 73. — Monocaulis peiv- 

 dula^ natural size. 



VOL. I. 



