SUMMARY 403 



DISCUSSION 



From what has been said on the structure and activities of Physalia, it will be seen that the outstanding 

 need is for further physiological and experimental work on living animals. This need is very apparent 

 where the functions of gas-secretion and digestion are concerned, but I would like to comment 

 particularly on the organization of the nervous and muscular systems. 



We know from observation of living specimens that a rapid, probably through-conducted, contrac- 

 tile response takes place throughout the organism following strong tactile stimulation, and that at 

 other times the members behave independently, but histological study reveals no differentiation of 

 the nerve plexus into two systems, one of which could serve for through-conduction, the other for 

 local conduction. Even more puzzling is the absence of nerves from three of the four float epithelia. 

 In the case of the ectoderm and endoderm of the saccus, the histological absence of nerves fits in well 

 with the apparent incapacity for active response, but in the case of the codon endoderm, where the 

 musculature must contract and relax in harmony with that of the ectoderm to produce the charac- 

 teristic postural changes, the absence of nerves is surprising. One can only suppose that the muscle 

 response here is a direct one, possibly to tensions set up in the mesogloea by contraction in the 

 ectoderm, possibly to forces negotiated via the ' pneumatic skeleton '. 



It is easy to make the mistake of attributing to siphonophores more refined physiological mechanisms 

 than their basically simple structure can allow. It must be borne in mind that there are few structures 

 here which could be called organs, that there are no muscles in the sense of groups of fibres or fibre 

 bundles having localized origins and insertions, and that there are no ganglionic aggregations of nerve 

 elements which could function as centres of direction and co-ordination. To think of the integration 

 of the organism in terms of motor versus sensory pathways, reflexes, pacemakers etc., is fraught with 

 danger. The nervous system is dispensed with completely in many regions, for example, the gas- 

 gland, which by analogy with vertebrates one would expect to be well innervated. 



One other topic calls for comment. In studying the histology of Physalia, one finds signs that the 

 large size of the animal sometimes places demands upon the component tissue elements which can be 

 met only with difficulty and with deleterious side effects. One recalls particularly the cytological 

 aberrations referred to on pp. 381, 382, some of which appear to be caused by excessive flattening of 

 the epithelia and mechanical stress. If the epithelia were able to proliferate in depth and to differentiate 

 into further cell types, such mechanical difficulties would not arise, but this ability would require the 

 existence of a third germ layer, that is a mesoderm. Some writers deny that the distinction between 

 mesoderm and mesogloea is a valid or significant one. However, mesoderm, as a source of cells, can 

 relieve the ectoderm and endoderm of many functions and allow an overall increase in tissue specializa- 

 tion. Mesogloea, however elaborate, provides no such benefits. 



SUMMARY 

 A. Behaviour 



(1) The float of Physalia responds actively to wind by adoption of a characteristic ' sailing posture ' 

 together with erection of the crest. Crest erection results automatically from an increased pressure on 

 the enclosed air ('pneumatic skeleton') whether caused by a general increase in muscular tone, 

 as in natural conditions, or by experimental manipulation. Observations on rolling behaviour 

 support the view that these movements represent attempts to adopt the sailing posture in conditions 

 of calm. 



(2) Muscular contraction, both in the float and tentacles, is stepped. 



