92 COMPARATIVE ANATOMY. 



aboral pole, to a definite cliaracter of tlie most simple form in the 

 Physemarife, and in Olynthus among the Ascones. In other Calci- 

 spongige, also, this simple form of body is retained, but more 

 considerable changes in its internal characters obtain. 



The most important changes in the form of the body are due to 

 the formation of colonies. Colonies of the most varied form (cormi) are 

 formed by budding or by incomplete division, the separate animals 

 (personae) of which are connected with one another in very various 

 ways, and may even partly or completely fuse with one another in 

 different ways. When these stocks are fused they often have the 

 appearance of single animals, and in proportion to the simplification 

 of their external form is the complication of their internal organi- 

 sation. The modification of the mouths of the colony aifects 

 their external form just as much as does this concrescence; the 

 mouths may be collected into groups, or united as one, or they 

 may completely disappear. 



The gi'eat variety of form in this division, which is due to the 

 causes here only indicated, may be still further modified by numerous 

 adaptations, due to their position in space. Nowhere in the Animal 

 Kingdom does the forni of the body appear to be so changeable as 

 in the Spongige, so that it is impossible to separate them into largo 

 divisions, to say nothing of species. 



§ 73. • 



In almost all the divisions of the Acalephce the body developed 

 from the Gastrsea-form is adapted to a sessile or fixed condition; 

 the stomachal cavity which is formed when this development com- 

 mences causes the organism to have, in all essential points, the same 

 simple character as it has in the corresponding stage in the life- 

 history of the Spougiaa. Processes, known as tentacles, are developed 

 on the anterior region of the body- wall, which encloses the stomachal 

 cavity ; they present to us the earliest indication of a differentia- 

 tion of the secondary axes, and therefore establish a well-marked 

 distinction between these forms and the Spongife. 



The Hydroida or Hydroid-Polyps (Hydriformes) are the lowest 

 of the Hydromedusaa. 



Tentacles are placed in many cases irregularly on the parts of 

 the body which surround the mouth (Coryne, Syncoryne, Cordy- 

 lophora), or their number maybe indefinite, even when the structures 

 are limited to definite zones of the body, and encircle the mouth in 

 the anterior region (Hydractinia, Eudendrium, Campanularia). As 

 the number of tentacles varies we cannot suppose that the secondary 

 axes are definitely differentiated. It is only in a few cases that they 

 are definitely expressed by the position of the tentacles (Stauridium). 



The free portion of the body, with its tentacles, becomes more 

 independent of the rest of the body, which forms a stalk, while the 

 aboral pole grows out into a stalk-like part, which carries the head, 

 and is distinguished as the " capitellum " or " hydranth." 



