9O MANUAL OF ZOOLOGY. 



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 Hydrozoon 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. As a rule, the 

 zooids of the trophosome are all like one another, or are 

 " homomorphic ; " but there are some cases (as in Hydrac- 

 tinia, and in the nematophores of the Plumularida) in which 

 some of the zooids of the trophosome are unlike the others. 

 The zooids of the gonosome, on the other hand, are normally 

 unlike, or are " heteromorphic," consisting of two or three 

 different sets of zooids, each with its special duty in the gener- 

 ative functions of the Hydroid Colony. 



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. 



In some few cases the hydrosoma is composed of a single 

 polypite only, as in the Hydrida and in some of the Corynida; 

 but usually there are several polypites united together by 



