HYDROID COLONIES— PORTUGUESE MAN-OF-WAR. 



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inner walls of the manubrium, and give rise to ciliated larvae which paaa 

 through the cavity of the umbrella into the open water, become attached, and, 

 by budding, produce new Hydroid colonies. Such colonies again produce 

 Medusoids, and so on. This process is a simple case of what is called 

 alternation of generations. The Hydroid form has been called in German 

 the "nursing" generation, as it does not itself produce the young, but 

 brings to perfection the individuals which, when free, become the reproduc- 

 tive generation. 



Although most Hydromedusse are the products of a Hydroid colony, and 

 their eggs develop again into such colonies, there are others which develop 

 direct from forms in every respect like themselves, i.e., the young ones no 

 longer pass through any attached Hydroid stage. On the other hand, there 

 are Hydroid colonies which never develop free raedusoid forms to secure the 

 dispersal of their progeny. 



Leaving the fixed Hydroid colonies and their free swimming members, we 

 come to the one remaining group of the Hydrozoa, the Siphonophora, which 

 consists of large Hydroid colonies, no longer attached, but 

 floating freely about. These are very complicated The Siphonophora. 

 creatures in which the Hydroid and Medusoid type of 

 body is so curiously changed as to be scarcely recognisable, and in which 

 also division of labour has reached an extreme point. 



The Siphonophora are exceedingly striking and beautiful objects, only to 

 be seen in tropical seas, where they delight the traveller by their exquisite 

 shapes and brilliant colouring. 



The colonies are not branched, but the many members are arranged round 

 a central stalk. The cavities of all the members communicate as in the 

 Zoophytes. The uppermost individual is turned into a kind of air-bladder for 

 floating the colony, and then come a number of individuals turned into 

 swimming-bells which drive the whole colony through the water. Beneath 

 these locomotory individuals come nutritive individuals, which are often 

 little more than stomach-tubes, reproductive individu- 

 als, frequently in clusters, and protective individuals, 

 arranged in such a way as to overlap the nutritive and 

 reproductive members of the colony. There are, 

 further, tentacular individuals which are usually long 

 filaments (sometimes called fishing lines). These are 

 richly provided with stinging batteries, their function 

 being to capture food for the colony and to defend 

 it from its enemies. 



In the simpler forms of Siphonophora the stalk is 

 long, and the members of the colony project from it 

 at intervals. Sometimes several pairs of swimming 

 bells follow each other for a short way down the stalk, 

 in other cases a single pair heads the colony. In 

 other forms the stalk is much shortened, and a central 

 air-bladder is surrounded by medusa-like swimming 

 bells, the other members of the colony being crowded 

 together beneath these. One of the best -known 

 Siphonophora is the Portuguese Man-of-War, illus- 

 trated in Pig. 8. This is one of the most specialised 



forms The large, crested, and exquisitely coloured air - bladder alone 

 appears above the waves. This air-bladder secretes its own gas, -"H 



Fig. 8— Tub PoRTOOui'sa 

 Man-of-Wak {Phi/satia). 



and has 



