HYDROZOA 51 



This Phylum is divided into four classes, (1) Hydro- 

 zoa, (2) Scyphozoa or Acalephse, (3) Anthozoa or Actinozoa, 

 (4) Ctenophora. 



CLASS I. HYDROZOA 



The simplest type of the Hydrozoa is the common 

 freshwater Hydra. In this the body has the form of an 

 elongated sac, about a quarter of an inch in length, and is 

 attached by one end, whilst at the other is the mouth 

 surrounded by a row of long processes, called tentacles. 

 The large undivided cavity in this sac, which opens into 

 the hollow tentacles above, is the ccelenteron. The whole 

 body is very contractile and constantly changing its shape. 

 Reproduction may take place in three ways, (1) by the 

 growth of buds, which ultimately separate from the parent, 

 (2) sexually, by the production of ova and spermatozoa in 

 the ectoderm, and (3), in rare cases, by fission. 



Other Hydrozoa consist of a number of individuals 

 (polyps or hydranths) similar to Hydra, but growing 

 together as a colony (fig. 12); all the individuals in such 

 cases are placed in living communication by means of a 

 tube-like extension from the base of each polyp ; this 

 common connecting portion of the colony is called the 

 camosarc (fig. 12, 5). Frequently the coenosarc is much 

 branched, giving rise to tree-like forms; it is usually 

 attached to some foreign object by a horizontal branching 

 portion. 



In such hydroid colonies the polyps are asexual, and 

 the reproductive elements are produced in another in- 

 dividual of a somewhat different character, known as 

 a medusa or gonophore: this arises by budding from the 

 hydroid (fig. 12, 9), and is often more or less bell-shaped, 



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