' CCELENTERATA : HYDROIDA. I 1 1 



distinguished as the " producing 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 nu- 

 trition 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 repro- 

 ductive. For the whole assemblage of the nutritive zooids of a Hydrozob'n 

 Professor Allman 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 gono- 

 some, however apparently independent, and though endowed with intrinsic 

 powers of nutrition and locomotion, can be looked upon as an "individual," 

 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 Hydractinia, and in the nematophores of the Plumularido>) in which 

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

 the gonosome, on the other hand, are normally unlike, or are "hetero- 

 morphic, " consisting of two or three different sets of zooids, each with its 

 special duty in the generative functions of the Hydroid colony. 



CHAPTER VIII. 



DIVISIONS OF THE HYDROZOA. 

 SUB-CLASS HYDROIDA.* 



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

 droida, the Siphonophora, the Lucernarida, the Graptolitida, and 

 the HydrocoraUina. 



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

 Hydrozoa which consist of an alimentary region or " polypite" 

 which is, typically, provided with an adherent disc, or " hydro- 

 rhiza" 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 means 

 of a common trunk or "coenosarc," as in most of the Corynida 

 and in the orders Sertularida and Campanularida. Further, 

 in the great majority of cases, the "hydrorhiza" is permanently 

 attached to some foreign object. 



The Hydroida comprise six orders viz., the Hydrida, the 



* For full details as to the morphology and physiology of the Hydroid 

 Zoophytes, the student should refer to the magnificent 'Monograph of the 

 Gymnoblastic Hydroids,' by Professor Allman (Ray Society). The student 

 may also consult the excellent ' History of British Hydroid Zoophytes,' by 

 the Rev. Thomas Hincks. 



