■96 Introduction to Animal Morphology. 



Order 5. Milleporidae — trophosome colonial, en- 

 crusting submerged materials in tropical and sub- 

 tropical seas ; massive or lamellar, often antler-like, 

 branching, calcified ; tentacles four ; ccenenchyma 

 abundant ; tabulae transverse. Only two genera sur- 

 vive : Millepora, with an irregularly tubular structure ; 

 and Heliopora, with tubular ccenenchyma and twelve 

 regular septa, forming coral-like masses. Possibly 

 the extinct Rugosa and Favositidae should be included 

 here [Agassiz). The reproductive zooids of all are 

 unknown. 



CHAPTER XV. 



HYDROZOA. 



Sub-class 2. Siphonophora [Eschscholiz] — Free, 

 oceanic, often bilateral or asymmetrical, with a 

 muscle-bearing, flexible caenosarc, whose proximal 

 end is dilated, containing an enlargement of the 

 somatic cavity, often acting as a hydrostatic apparatus, 

 and usually bears barren, medusiform zooids, spe- 

 cialized into swimming bells (nectocalyces). The ova 

 are fertilized directly, and the germinal vesicle does 

 not disappear in the early stages of development. 

 The gonophores are medusiform, borne on the 

 peduncles of the polypites. About 120 species exist, 

 divided into two orders : — 



Order i. Calycophoridse [Leiickart] — having the 

 hydrosoma propelled by nectocalyces, each of which 

 resembles the bell of a planoblastic medusa without a 



