C(ELENTERATA. HYDROZOA. 39 



compartments by partitions (mesenteries). The simplest 

 form of hydrozoan is the common freshwater Hydra. 

 In this the body consists of an elongated sac, at one end 

 of which there is an opening, serving as the mouth, the 

 animal being attached by the opposite end. The interior 

 of this sac is hollow and undivided, it is produced above 

 into a row of tentacles surrounding the mouth ; the whole 

 body is very contractile and constantly changing its shape. 

 Multiplication takes place (1) by the giving off of buds 

 which ultimately separate from the parent, (2) by fission, 

 and (3) sexually, by the production of ova and spermatozoa. 

 The sexual elements are developed in the ectoderm. In 

 many hydrozoans however, the sexual elements are pro- 

 duced in another individual (medusa), so that there is an 

 alternation of a sexual with an asexual generation. In a 

 large number of forms there are no hard parts what- 

 ever, others possess a skeleton which is produced by 

 the ectoderm, and may be chitinous as in Sertularia, or 

 calcareous as in Millepora. A simple hydrozoan, like 

 Hydra, is known as a polyp, but many forms are compound, 

 consisting of a number of polypites united together. All 

 the hydrozoans, except Hydra and Cordylophora are 

 marine. They may be divided into five sub-classes, 

 (1) HydromedusaB (or Hydroidea), (2) Scyphomedusse, 

 (3) Siphonophora, (4) Graptolitoidea, (5) Stromatoporoidea. 

 The Hydromedusae includes such modern genera as 

 Tubularia and Sertularia. Although there is frequently 

 a chitinous or calcareous skeleton, this sub-class is but 

 poorly represented in the fossil state. The living form 

 Hydractinia is found in the Cretaceous and later forma- 

 tions. Probably the genera Dendrograptus found in the 

 Cambrian and Ordovician, and Dictyonema which ranges 

 from the Cambrian to the Devonian, also belong to this 



