13S ZOOLOGY. 



a chitinous or horny covering, denominated the poly- 

 pary. This covering does not enclose the polypites in 

 the club-hydras, as it terminates at their bases. 



195. The Tubularia is an example of this group. It 

 consists of a number of horny tubes which are attached 

 to stones or shells. These tubes are not branched, but 

 are often interlaced with one another. A single polypite 

 is situated at the extremity of each. The tubes are filled 

 with a red jelly-like substance. The polypites are also 

 of a red colour. They are provided with two sets of 

 tentacles, one short row being placed round the mouth, 

 and a longer row near the base. 



196. Development. In the compound Hydrozoa, the 

 buds, instead of being separated from the original stock, 

 as in the fresh-water hydra, remain permanently attached 

 to it, and soon begin to develop other buds. In this way 

 a colony is formed. There are often two different sorts 

 of zooids. One kind are without reproductive organs, 

 their function being merely to procure food for the 

 colony. These are denominated alimentary zooids, 

 and taken collectively, are called the trophosome (Gr. 

 trepho, I nourish ; soma, body). These go on increasing 

 by budding; but, at a certain stage in the life of the 

 colony, a second set of buds is produced, furnished with 

 ova and sperm cells. These buds are termed gonophores 

 (Gr. gonos, offspring ; phero, I bear). These gonophores 

 are often much larger than the alimentary zooids, and are 

 of three different kinds. 



197. One kind consists of a protuberance formed by the 

 ectoderm and endoderm. They are closed without, but 

 maintain an internal connection with the ccenosarcal 

 fluid. They produce fertile ova. 



198. A second kind are bell-shaped discs, which are 

 attached to the polypary by the base, the cavity of the 

 bell being turned outwards. There is a kind of handle 

 within the cavity of this bell, occupying the place of the 

 clapper, and termed the manubrium (Lat. manubrium, a 

 tandle.) From the point where the manubrium is 



