372 DISCOVERY REPORTS 



BEHAVIOUR 

 i. Introduction: individuality in Siphonophores 

 It is customary to speak of siphonophores as colonies consisting of modified polypoid and medusoid 

 individuals, all attached to a common stem. 



This simple picture is satisfactory enough where the medusoid individuals are concerned. In the 

 colonial Hydrozoa nectophores and gonophores are always easy to recognize as medusoids, both in 

 their manner of development and in their mature structure. The remaining parts of a colony are not 

 always so easy to classify. For example, in both the sessile Corymorpha and its pelagic relative Velella, 

 the sexual medusoids are borne upon protuberances from the body-wall which are alike and homo- 

 logous in every respect, except that in Corymorpha they are mouthless, while in Velella they have 

 mouths and ingest food. In Velella they are known as 'gonozooids'. Taking a rigid view of the 

 concepts of the colony and the individual, two interpretations are possible for this state of affairs. 

 Either the gonozooids represent true individuals (as their name implies) and function as such in 

 Velella while becoming reduced in Corymorpha ; or they have originated in phylogeny from hydranth 

 outgrowths, and such individuality as they have acquired is secondary. If the first interpretation is 

 correct, then Corymorpha must be regarded as a reduced colony. If the second is correct, then the 

 structures in question should not be called gonozooids, unless it is made clear that they have acquired 

 their individual status secondarily. The latter interpretation is preferable and it leads to the conclusion 

 that individuality can be complete or partial ; that it can probably also be lost or gained ; that it is, in 

 fact, an unreliable concept. 



If the siphonophores are considered from the functional point of view, the distinction between the 

 colony and the individual becomes even harder to define. In many siphonophores, one must recognize 

 that a new, communal individuality has emerged from the ancestral assemblage. The organism acts 

 like a well-integrated individual. Whatever their origins, the component parts have now achieved 

 the status of organs in an individual. In a certain sense, the siphonophores represent a method of 

 escaping from the limitations of the diploblastic pattern. Another more successful method of escape 

 involved the development of a third germ layer. In the Triploblastica, organ systems develop within 

 the individual. In the Siphonophora, individuals become organs. 



The problem now to be considered concerns the extent to which Physalia has progressed beyond the 

 condition of an assemblage of autonomous individuals toward the status of a new individuality. 



2. Behaviour of the float 

 The outline of the float (seen from above) is shown in Text-fig. i. The main groups of appendages are 

 situated around the bulge (b). When the float is being driven before the wind the drag is concentrated 

 in this region and, as a result, the float automatically assumes the orientation shown. The shape of the 

 float is such that when orientated for sailing it does not move directly down-wind, but to one or other 

 side of the down-wind direction, according to the mirror-image dimorphism (Woodcock, 1944, 1956; 

 Totton and Mackie, 1956).* 



In adopting a sailing posture the float responds actively by erection of the crest or sail (s); at the 

 same time the apical pore-end (a) curves round toward the windward side. Bigelow (1891) found that 



* In my thesis (submitted July, 1956, at Oxford) I suggested that the advantages inherent in dimorphism are not to be 

 explained by reference to local phenomena such as distribution of Sargasso weed, islands, etc., but should be thought of in 

 terms of world distribution. In any given ocean one dimorphic form will presumably be better fitted to survive than the 

 other, but it will not always be the same form, and, for the species as a whole, it is not important which form has the 

 advantage. The important thing must be that by virtue of the dimorphism the species is fitted for life in any ocean. 

 Woodcock (publ. August, 1956) adopts a rather similar view. 



