18 NATURAt HISTORY OF 



cavity, and the reproductive organs are external ; and 

 II., the AcTiNOZOA or Anthozoa, in which the wall of 

 the digestive sac is separated from that of the somatic 

 cavity by an intervening space subdivided into chambers 

 by a series of vertical partitions, on the faces of which 

 the reproductive organs are developed. {Greene, 

 " Coelenterata.") 



The Htdrozoa are divided into three orders, viz. : 

 I., the Hydroida ; II., the Siphonophora; and III., the 

 DiscoPHORA or LucERNARiD^. The first of these orders 

 only will be dealt with in this book, and will be described 

 more fully in the part specially devoted to it.* 



The AcTiNOZOA or Anthozoa are divided into two 

 orders, namely, the Zoantharia and the Alcyonaria, 

 the distinguishing characteristics of which will be 

 afterwards described. 



The preceding groups are separated from the re- 

 mainder of the animals usually included in the expression 

 "Zoophytes" by three sub -kingdoms, the Echino- 

 dermata, the Articulata, and the Vermes, the two 

 latter of which are sometimes comprised in one sub- 

 kingdom, called the Annulosa. 



The PoLYzoA, as the remaining group of the Zoo- 

 phytes is called, constitute a class of the sub-kingdom 

 MoLLuscA, which consist of soft-bodied, unsegmented 

 animals, usually provided with an external skeleton, 

 and having a nervous system in the form of one to 

 three principal pairs of ganglia. The Polyzoa are 



* Those who wish to study the Siphonophora and Discophora 

 are referred to Greene's " Manual of the Ccelenterata," 1875 ; to 

 Huxley's "Monograph of the Oceanic Hjdvozoa," Eay Society, 

 1859 ; Forbes' " Monograph of the British Naked-eyed Medusae," 

 Hay Society, 1848 ; and Haeckel's " System der Medusen," 1879. 



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