FOEM OF BODY OF CCELENTEEATA. 95 



the primitive mode. It is found oil this region in all divisions of the 

 Hydroid-Polyps. The stocks of the Corynidas have buds distributed 

 over the hydranth. They are frequently placed between the 

 tentacles. In Peunaria they are found within the circlet of tentacles ; 

 and on the same place in the Tubularias, where they are always placed 

 in some quantity on a common stalk, forming groups like grapes, or 

 ears of corn. Gemmation on the body of the Hydroida is, in many 

 cases, accompanied by a degeneration. In many Canipanulariae, 

 Hydractinias, and others, the proliferating individual gives up its 

 share in the duty of feeding the stock, as is expressed by the 

 diminished size of its tentacles and stomachal cavity. The animal 

 stock is therefore composed of nutritive and proliferating persons, 

 the latter again bearing the buds or generative persons. This is 

 the way in which the Dimorphism of Persons arises in these 

 stocks, and this passes into Polymorphism, in consequence of a 

 number of the nutritive persons undergoing still further changes 

 (Hydractinias) . 



The proliferating persons of a colony present various degrees of 

 degeneration. In the most extreme case a portion only of the 

 individual which bore the buds remains after they are developed 

 (e.g. in many Campanularias) . The complete degeneration of the 

 proliferating person causes the geinmge to arise from any part of the 

 common stock, without any relation to a Hydroid person. 



In the higher divisions of the Medusiformes, relations to the 

 Hydroida are lost. Although there are many considerable complica- 

 tions in the mode of reproduction (see below, Generative Organs), 

 yet, so far as is yet known, there is among the Trachynemidas, 

 yEginidas, and Geryonidaa no return to the hydroid form; and it 

 is doubtful whether any such relation does obtain. 



§ 75. 



The division of labour, which in Hydroid-Polyps is essentially 

 limited to the nutritive and generative functions of the persons 

 united into a colony, is extended over a larger series in the Sipho- 

 nophora ; and we met therefore with great variety in the form of 

 the component parts of the colony. Division of labour thus leads 

 to Polymorphism of the persons. They all follow the medusiform 

 type, which is developed to a greater or less extent. When it 

 is distinctly developed, the fundamental form which shows itself 

 in the medusa-buds of the Hydroid-Polyps predominates, so that 

 it is clear that both groups have had a common origin. The 

 Siphonophora, therefore, appear to be swimming Hydroid colonies, 

 all the persons of which have passed into the medusa form ; a change 

 which is complete in the generative persons only of the Hydroid- 

 Polyps. The separate persons of the colony of the Siphonophora 

 are developed on a common contractile stem, which in most forms 

 represents the axis of the stock, and around which the persons 

 which function as organs of the whole colony appear to be arranged. 



