306 COELENTERATA — HYDROZOA chap. 



kinds of zooids : a gastrozooid with a trumpet-shaped mouth armed 

 with nematocysts, a long dactylozooid provided with a series of 

 tentilla, and a rudimentary gonozooid bearing numbers of male 

 or female medusiform gonophores. These three kinds of zooids 

 are partially covered and protected by a bent shield -shaped 

 phyllozooid or hydrophyllium. 



Each of the cormidia is unisexual, but the colony as a whole 

 is usually hermaphrodite, the male and female cormidia regularly 

 alternating, or the male cormidia being arranged on the necto- 

 calycine half and the female cormidia on the opposite half of the 

 stolon. 



The families of tlie Calycophorae are : — 



Fam. 1. Monophyidae.— In this family there is a single 

 conical or mitre -shaped nectocalyx. The cormidia become de- 

 tached as free-swimming Uiidoxia or Ersaea forms. 



Sub -Fam. 1. Sphaeronectixae. — The primary nectocalyx 

 persists throughout life — Mojwjjhyes and Sphaeronectes. 



Sub -Fam. 2. Cymbonectinae. — The primary nectocalyx is 

 thrown off, and is replaced by a secondary and permanent necto- 

 calyx — Cynilonectcs, Muggiaea, and Doramasia. 



Fam. 2. Diphyidae, — The primary mitre-shaped nectocalyx 

 is thrown off and replaced by two secondary rounded, prismatic, 

 or pyramidal, heteromorphic nectocalyces. 



This family contains several sub-families, which are arranged in 

 two groups : the Diphyidae Oppositae, in which the two secondary 

 bells are opposite one another, and do not exhibit pronounced 

 ridges ; and the Diphyidae Superpositae, in which one of the 

 two secondary nectocalyces is situated in front of the other, and 

 each nectocalyx is provided externally with very definite and 

 often wing -like ridges. In all the Diphyidae Oppositae the 

 cormidia remain attached, whereas in most of the Dipliyidae 

 Superpositae they become free-swimming, as in the Monophyidae. 



The sub-families of the Diphyidae Oppositae are : — 



Sub-Fam. 1. Amphica1!Y0N1NAE. — One of the two secondary 

 nectocalyces becomes flattened above to form a shield, and at the 

 same time its sub-umbrellar cavity is atrophied, and its radial 

 canals reduced. Mitroj^hgcs, Atlantic Ocean. 



Sub-Fam. 2. Prayinae. — The colony exliibits a pair of large, 

 obtuse nectocalyces, with a relatively small sub-umbreUar cavity. 

 Praya, Mediterranean and Atlantic. 



