78 MANUAL OF ZOOLOGY. 



applied to the horny or chitinous outer covering or envelope 

 with which many of the Hydrozoa are furnished. These terms 

 have also not uncommonly been applied to the very similar 

 structures produced by the much more highly organised sea- 

 mats and their allies (Polyzoa), but it is better to restrict their 

 use entirely to the Hydrozoa. 



Zooids. In continuous development the partially inde- 

 pendent beings which are produced by gemmation or fission 

 by the primitive organism, to which they remain permanently 

 attached, are termed " zooids." 



In discontinuous development, where certain portions of 

 the " individual" are separated as completely independent 

 beings, these detached portions are likewise termed " zooids ;" 

 that which is first formed being distinguished as the "pro- 

 ducing zooid," whilst that which separates from it is known 

 as the "produced zooid." In a great number of Hydrozoa 

 there exist two distinct sets of zooids, one of which is destined 

 for the nutrition of the colony, and has nothing to do with 

 generation, whilst the functions of the other, as far as the 

 colony is concerned, are wholly reproductive. For the whole 

 assemblage of the nutritive zooids of a Hydrozob'n Professor All- 

 man has proposed the term " trophosome," applying the term 

 " gonosome " to the entire assemblage of the reproductive zooids. 

 In such Hydrozoa, therefore, as possess these two distinct sets 

 of zooids, the " individual," zoologically speaking, is composed 

 of a trophosome and a gonosome. It follows from this that 

 neither the trophosome nor the gonosome, however apparently 

 independent, and though endowed with intrinsic powers of 

 nutrition and locomotion, can be looked upon as an " indi- 

 vidual," in the scientific sense of this term. 



CHAPTER VIII. 



DIVISIONS OF THE HYDROZOA. 

 SUB-CLASS HYDROIDA. 



THE Hydrozoa are divided into four sub-classes, viz. the Hy- 

 droida, the Siphonophora, the Lucernarida, and the Discophora. 

 SUB-CLASS I. HYDROIDA. This sub-class comprises those 

 Hydrozoa which consist of an alimentary region or " polypite" 

 which is provided with an adherent disc, or " hydrorhiza" and 

 prehensile tentacles. 



