52 HYDROZOA 



and may become detached from the colony, or it may be 

 less perfectly developed and remain attached ; at the inner 

 edge of the bell is a shelf-like fold, the velum. The 

 generative cells are of ectodermal origin, and from them 

 the hydroid develops. 



Hydra possesses no hard parts, but in other forms 

 an external skeleton composed of chitin or of carbonate 

 of lime is secreted ; it commonly forms a tube-like sheath 

 around the coenosarc and is called the perisarc (fig. 12, 6). 

 In one group the perisarc is produced at the base of 

 each polyp into a cup-like structure or hydrotheca (fig. 

 12, 7), into which the polyp can retract. The gonophores 

 may also be protected by a chitinous capsule called the 

 gonotheca or gonangium (fig. 12, 10). The vertical branch- 

 ing part of the coenosarc together with the perisarc 

 around it, is called the hydrocaulus ; the horizontal root- 

 like portion and its perisarc form the hydrorhiza. 



The principal characters which distinguish the Hydrozoa 

 from the other Coelenterates are : the coelenteron being 

 undivided by radial partitions or ridges ; the absence of a 

 digestive tract projecting into the coelenteron; the usual 

 occurrence of an asexual (hydroid) generation alternating 

 with a sexual (medusoid) generation ; the medusa having 

 a velum ; the ova and spermatozoa being derived from the 

 ectoderm. 



Nearly all the Hydrozoa are marine. They are divided 

 into eight Orders, of which five occur fossil: — (1) Gymno- 

 blastea, (2) Calyptoblastea, (3) Graptolitoidea, (4) Hydro- 

 corallina, (5) Stromatoporoidea. 



ORDER I. GYMNOBLASTEA 



The Gymnoblastea have no hydrothecse into which the 

 polyps can retract ; gonangia (gonothecse) are also absent. 



