SUPPLEMENTARY. 



There still remain for discussion in connection with the general zoology of the Hydroida 

 some subjects which, in strict sequence, ought to be included under the head of Morphology. 

 It has been deemed, however, more convenient to defer their consideration, and they are 

 accordingly here brought together in a separate section supplementary to the first part of the 

 Monograph. 



CLASSIPICATION. 



An important reform in zoological classification was introduced when Frey and Leuckart 

 separated as a distinct sub-kingdom, under the name of Ccelenterata, certain animal forms which 

 had been previously combined with the Ecliinodcrmata in the construction of the Cnvierian primary 

 group of the RADiArA.' In the establishment of the Sub-kingdom Coslenterata the German 

 zoologists recognised the importance of a fundamental type of organization in which the general 

 somatic cavity communicates freely with the outer world through the mouth. 



The Ccelenterata admit of subdivision into two classes, in accordance with the presence or 

 absence of a stomach-sac differentiated from the general body-cavity though freely opening into it. 

 For these two classes the names of Hydrozoa and Actinozoa assigned to them by Huxley 

 may be accepted as sufficiently convenient. Each of these classes is further divisible into orders, 

 and one of the orders of the Hydrozoa is constituted by the Hydroida, the group to which 

 the present Monograph is devoted. 



The characters which I regard as essentially distinguishing the Hydroida are expressed in 

 tlie following table, from which the relations of the Hydroida to the other ccelentcrate orders will 

 be at once apparent : 



^ Frev uud Leuckart, " Beitrage zur Kenntniss wirbelloser Tliierc," 1847. 



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