108 COMPARATIVE ANATOMY CHAP. 



stocks formed of individuals whose gastric cavities remain connected, 

 like those of the Reef Coral, by characteristic arrangements of the canal 

 system. The above illustrations (Fig. 82, A-C) show diagrammatically 

 the manner of gemmation and the beginning of stock formation in 

 different Alcyonaria. 



In the Alcyonaria also, division of labour takes place, with the 

 resulting dimorphism or polymorphism of the persons (zooids). Thus, 

 side by side with the normal persons there are other persons without 

 tentacles and with septa reduced in number (two) whose chief function 

 is the taking in of water into the canal system. 



XIII. Organisation of the Siphonophora. 



It is most suitable for our purpose here to describe the structure 

 of the Siphonophora, as it can only be explained and understood by 

 help of the phenomena of asexual reproduction by means of gemma- 

 tion, of stock formation, and of the division of labour. The Siphono- 

 phora have actually been long considered by most investigators as 

 polymorphic animal stocks, although zoologists had not agreed as to 

 the significance of the separate parts. 



The following description corresponds in general with the views 

 recently put forth by Haeckel. 



In the order of the Siphonophora two animal groups have till now 

 been united, which, apart from the fact that both are Medusa- stocks, 

 have nothing in common, and, in any case, have quite different origins. 

 We shall therefore treat of these two groups (1) the Siphonanthe and 

 (2) the Diseonanthe separately. 



I. The Siphonanthe. These may be conceived of as colonies of 

 Craspedote Medusce by comparing their whole body with a Craspedote 

 Medusa on whose gastric peduncle numerous young Medusce have 

 arisen by gemmation, somewhat in the same way as in Sarsia 

 Siphonophora. While, however, the mother Medusa, of the Sarsia is 

 radially constructed and all the daughter Medusce are like each other 

 and like the mother, the mother animal of a Siphonanth which is 

 recognisable in its young or larval stage is a much metamorphosed 

 Medusa. Its disc is mostly changed into an ' air vesicle, it possesses 

 only one tentacle (which also occurs in the Craspedotci), and its gastric 

 peduncle is lengthened out into the generally very long " stem " of 

 the Siphonanth. The daughter Medusce, budding from the stem, are 

 neither like one another nor like the mother animal. They divide 

 between them the general work, and are consequently variously 

 modified to suit their special functions. 



If we now more closely consider the body of a Siphonanth, we 

 must first bear specially in mind those parts which can be compared 

 with parts or organs of the mother Medusa of a proliferous Craspedote. 



A. The pneumatophore or swim -bladder (Fig. 83) lies at the 



