THE CLASSIFICATION OF ANIMALS. 131 



very large vacuoles, retain an amoeboid 6haracter. When 

 food has been swallowed, they reach out pseudopodia, or finer 

 processes called flagella, to catch solid particles. The latter 

 are ingested by the substance of the cells, just as occurs in 

 A-nKxba. This process is spoken of as "intracellular diges- 

 tion." 



Position of the type in its group. The organism which has 

 been chosen as a type is in this case one of the simplest 

 forms presented by its group ; the higher forms present a 

 greater complexity of structure, and may possess nerves and 

 sense-organs. From many of its group it differs in not being 

 a permanently fixed form ; it does not, however, move with 

 much freedom. In addition to the power of sexual reproduc- 

 tion, it retains the power of asexual reproduction by budding, 

 a process which is very characteristic of the Coelenterata. The 

 buds of Hijdra are thrown off ; in other cases, where they 

 remain attached, colonial forms are produced, such as corals 

 and corallines. By the alternation of forms which are only 

 capable of budding with special forms, which are capable of 

 sexual reproduction, " alternation of generations " may be 

 produced (p. 150). But in Hydra, ovring to the fact that the 

 buds are set free, no colony is formed, and no " alternation 

 of generations " takes place. In this respect. Hydra is 

 therefore not typical even of the order to which it belongs 

 {Hijdromedui'w). 



Habits. The Hydra is a minute fresh-water form, found 

 standing on water-weeds, in ponds and ditches. It catches 

 small organisms with its tentacles, benumbs them with the 

 sting of its nematooysts, and swallows them. 



Development. In Hydra, as in other Metazoa, the fertilized 

 egg-cell, produced by the union of a spermatozoon with an 

 ovum, divides by two, and then again and again by two, until 

 a mass of cells is formed. Varying accounts have been given 

 of the development of Hydra in its further stages ; but the 

 development of allied forms presents a ciliated larva of elon- 



