CCELEN TERA TA. 165 



but, besides, most forms have the power of forming buds 

 which grow into new individuals, sometimes like, in 

 others greatly different from, the parent ; the buds some- 

 times remaining attached, as often separating from the 

 parent, With very few r exceptions the Ccelenterates are 

 marine. The phylum is divided into three classes: Hy- 

 drozoa, Scyphozoa, and Ctenophora. 



CLASS I. HYDROZOA. 



The Hydrozoa are mostly small, and among them 

 colonial forms predominate. They are distinguished 

 from the other classes bv the absence of an inturned 



\j 



oesophagus and of well-developed radial partitions divid- 

 ing the digestive cavity. The tentacles also are solid. 

 In their life-history there are frequently some wonderful 

 changes, and to describe these we may follow the life- 

 cycle of Pennaria, described in the laboratory work. 



From the egg there hatches out a little oval, free-swim- 

 ming embryo, which soon attaches itself by one end to 

 some submerged rock, while a mouth breaks through at the 

 other, and tentacles grow out around the sides of the body. 

 When a mouth is formed feeding and growth are possible. 

 As the animal grows larger little buds appear on the sides, 

 and these, forming mouths and tentacles, grow into hy- 

 dranths like the parent. These buds never become free, 

 but the whole colony thus formed has a common digestive 

 tube by which all are connected. On the outside a tubular 

 horny protecting sheath, the perisarc, is developed. After 

 a while buds appear on the sides of the hydranths, and 

 these have a much different history, for they develop into 

 free-swimming jelly fishes. At other times the medusa 

 buds may develop from the stalk (fig. 14, ink) or from 

 the root-like stolons, which connect the parts of the colony 



