74 HYDROZOA AND POLYZOA. 



These are the simple members of the class, but many others 

 are compound, consisting of a stem with a sort of root, by which 

 they attach themselves to stones, etc., and branches on which cells 

 develop at intervals, each of these cells resembling the simple 

 forms just described in having a mouth, stomach, and tentacles 

 round the mouth, but differing in being all connected into a 

 compound structure, through which circulates a fluid, formed and 

 elaborated by such of the individual polyps (as the individual 

 cells are called) as are in active life at any particular time. This 

 common, connecting substance often develops on its outside a 

 horny tissue, which firmly supports and holds together the 

 delicate and soft substance forming the polyps, and being light, 

 elastic, and strong, allows the waves to bend it gracefully back- 

 ward and forward without injury. It is this horny or chitinous 

 external skeleton which remains when life has departed from the 

 organism. 



It may be useful next to explain why animals so different in 

 structure are often exhibited together, and thus many persons to 

 whom they are new are led to confound the one with the other. 



All the older authors, including Ellis, called them Zoophytes, 

 or animal plants, not recognizing their anatomical differences, but 

 judging from their external characters that they were all very 

 nearly related ; and even placing with them some vegetable 

 growths called Corallines, which only bore the most superficial 

 likeness. 



What are known as Corallines are a family of Algae which are 

 stiffened by a deposition of chalk in their cells ; but in the last 

 century many of the Hydrozoa, and Polyzoa also, were called 

 Corallines by Ehis, (whose work is dated 1755,) and by other 

 writers also. ElHs's work gives most faithful descriptions and 

 exact illustrations of these objects ; and he has a happy way 

 of naming in famihar English most of them — such as "Snake 

 coralline," " Bull's horn coralline," " Goat's horn coralline," 

 " Coat-of-mail coralline," etc. etc. 



But while the forms of the compound polypary or polyzoary * 

 bear a close resemblance one to the other, and while it is common 

 to both classes to have tentacles round the orifice by which the 

 food is admitted, there remain the important differences of 

 structure previously described, and the fact that while both 

 classes have tentacles, in the Hydrozoa the object of these 

 tentacles is to grasp the food and bring it to the mouth, while in 

 the Polyzoa the same end is gained in a very different manner. 

 The tentacles of the latter do not seize the food, but while they 



* Polypary is the term applied in the case of compound Hydrozoa, and 

 polyzoary in the case of compound Polyzoa. 



