Fossil Corals i — Lower Silurian Roclcs o/* Canada. 117 



In Fig, 1 is seen an ideal repre- 

 sentation of a Hydra, a minuto 

 fresli water animal remotely related 

 to the coral ]juilding, Polypi. * It 

 consists simply of a slender tubo- 

 like sack attached at one end to 

 some solid object in the water, suck 

 as a stone, twig, or floating piece 

 of wood, and having at the other 

 extremity a small opening sur- 

 rounded with several thread-like 

 tentacula. These parts constitute 

 the whole animal. There are no 

 viscera of any kind, heart, lung's^ 

 Vi^A, Ideal figure of a Hydra. blood vessels, or nerves within. — 

 The animal is simply an empty sack, with a mouth. Into this mouth is 

 drawn by the tentacula, various microscopic animalcula^, which happen 

 unluckily to venture Avithin their reach. Once witliin, they are soon digested 

 into a liquid which is absorbed iuto the walls of the sack, and contribute to 

 the nourishment and growth of the Hydra. The young seem to grow of their 

 o^Ti accord out of the sides of the parent. They '' appear at first as knob- 

 like protuberances from the body of the Hydra, they gradually increase in 

 6ize and come to present something of the form of the parent ; an apertuix; 

 is tlien seen at the free extremity, and around this, tentacula Ijegiu to sprout 

 The young during their growth are like so many buds upon the sides of the 

 original stock, and the hollow part of each communicates with the internal 

 cavity of the old one, from which they are fed. Even after the tentacula of 

 the bud are sufficiently developed to enable it to obtain food for itself, the 

 communication remains open for a time, as appeai-s from the fact that either 

 of the stomachs is distended when the other is fed. As the bud, however, 

 advances towards completeness, the aperture contracts, and is at last oblite- 

 rated ; the stock itself gradually becomes more slender, and is at last broken 

 by the slightest effort of either the old or the young Hydra, and the latter 

 is then set free, and after roaming through the water for a time attaches 

 Itself to a twig or stone and commences life and the rearing of a family on 

 its own account. There is no distinction of sexes, and what is more aston- 

 ishing the Hydra may be cut into pieces and each minute fragment will 

 grow into a new and perfect Hydra, and produce youug. f 



The Hydra is not a true coral animal, and has no hard parts. The 

 reef building animals are marine, and a little more complicated in structure. 

 If we were to imagine a small additional sack hanging down inside of the 



* Polyp, plural, Polypi. The general designation of coral animals, from' 

 the Greek, (polus,) many, and (po2cs,) foot ; the many tentacles of the Polyp being 

 at first considered the feet. 



t See Dr. Carpenter's Principles of Comparative Physiology, Hydra, iA 

 iadex. 



