STUDIES ON PHYSALIA PHYSALIS (L.) 



PART 2. BEHAVIOUR AND HISTOLOGY 

 By G. O. Mackie 



(Dept. of Zoology, University of Alberta) 



(Plates XXVI to XXVIII, text-figures 1-6) 



INTRODUCTION 



The morphological complexity of the siphonophores is well known. It has engaged the attention 

 of a great number of distinguished biologists, and it is only comparatively recently that the true 

 interrelationships of the group have been satisfactorily established. However, preoccupation with 

 gross morphology has led most workers to neglect certain important aspects of siphonophore organiza- 

 tion, particularly in the fields of physiology and behaviour. 



The group is unique amongst the Coelenterata in the capacity shown by some of its species to 

 secrete gas into a float and to regulate density (and hence to achieve vertical migration) by adjusting 

 the amount of enclosed gas; and yet, in recent times only one worker (Jacobs, 1937) has investigated 

 this phenomenon. The small size and delicacy of most siphonophores makes them exceptionally 

 difficult animals to handle, but, if the various technical difficulties could be overcome, the rewards of 

 a thorough physiological investigation would certainly be rich. Certain members of the Calycophora, 

 such as Hippopodius, can perform density regulation without the aid of a gas-filled float. The mechanism 

 for this is completely unknown. Furthermore, there is virtually no published information on the 

 extent of nervous co-ordination between the different parts of a siphonophore colony. These are but 

 two of the outstanding problems in the realm of functional organization. 



An investigation of the behaviour and reactions of the siphonophores was begun by the author in 

 1954, but it soon became clear that the significance of the behavioural findings would be hard to 

 establish unless more was known of the histology. Although we are still far from achieving a full under- 

 standing of the structural and functional organization of the siphonophores, it has been possible to make 

 some progress by means of this twofold method of investigation. In the following account, an attempt 

 will be made to describe the chief activities of one siphonophore, Physalia, together with the extent 

 of co-ordination between the different parts and the microscopic structure of the component tissues. 



The material used in the investigation came from three sources. The bulk of it was collected at 

 Lanzarote in the Canary Islands during the spring of 1955, and the behavioural observations were made 

 at that time. Some material was fixed for the author by Miss Elaine Robson from a specimen captured 

 near Plymouth in November 1954. Finally, some material from the' Discovery 'collections was examined. 



The author wishes to acknowledge his particular debt to Mr A. K. Totton, who conceived the idea 

 of a joint expedition to the Canary Islands and whose advice has been a great help, both then and 

 since. Certain of the results reported here were incorporated in the author's Doctor's thesis, which 

 was presented, under the supervision of Dr W. Holmes, at Oxford in 1956. It is a pleasure to acknow- 

 ledge the help of Professor Hardy, Dr Holmes and others at Oxford, and of the late Professor R. B. Miller 

 in whose department at Edmonton the work was completed. The expenses of the project were met by 

 a Research and Maintenance grant from the Department of Scientific and Industrial Research (which 

 included an additional sum of money for the Canary trip) and by grants from the General Research 

 Fund of the University of Alberta, and from the National Research Council of Canada. 



