38o DISCOVERY REPORTS 



Stained preparations were mounted in Canada balsam or in some other alcohol- or xylene-miscible 

 medium. As a check on the amount of shrinkage caused by dehydration and clearing, control strips or 

 gelatin sections were mounted in an aqueous medium (usually Farrant's) where shrinkage is negligible. 

 Measurements of cellular and epithelial dimensions were made on preparations which had never been 

 subjected to alcohol higher than 70%. 



3. The muscular system 

 In the tentacles, gastrozooids, palpons and float of Physalia, both ectodermal and endodermal muscle 

 fibres are present. As in Hydra, the endodermal fibres run in a circular direction, the ectodermal in 

 a longitudinal. 



In the tentacles and codon, the ectodermal muscle is very strongly developed. The ridges of meso- 

 gloea which support the muscle fibres are thrown into deep folds, particularly when the organism as 

 a whole is contracted. There appear to be no radially orientated muscle fibres in any region. The 

 endodermal system is less well-developed than the ectodermal, except in the codon. There is no 

 endodermal muscle in the medusoid members, and the ectodermal system is only properly developed 

 in the asexual nectophores. The subumbrellar muscle of these members has been examined in several 

 specimens, but no striations are visible. This probably indicates that the medusoids examined, though 

 the most advanced specimens obtainable from attached gonodendra, were not yet mature when fixed. 



The musculature of the float has been studied in greater detail than that of other regions because 

 it is technically easier to prepare. However, muscle throughout the animal shows the same histological 

 characteristics. 



A longitudinal section through a young Physalia is given by Okada (fig. 156D in Hyman, 1940). 

 The inner chamber of the float (the saccus) develops as an invagination, the region of invagination 

 becoming almost occluded in later life ; the only trace of it is the apical pore. Okada's section does not 

 go directly through the apical pore but slightly to one side of it, so that the opening from the inside 

 of the saccus to the exterior is not shown. In actual fact, the tissues never grow together in this 

 region, and the pore is not obliterated. It is sometimes possible to squeeze out a bubble of air, but 

 only by vigorous pressure. Normally the pore is tightly constricted and it is unlikely that in natural 

 conditions any leakage of air takes place. There is no other opening out of the float chamber, such as 

 occurs in forms like Physophora (Leloup, 1941). 



The saccus, being an invagination of the codon, has the same tissue layers as the latter, but they 

 are ' inside out '. The ectoderm of the saccus is the innermost layer of the float. Like the ectoderm of 

 the codon, it secretes a chitinous cuticle, but this has the special title of ' pneumatocyst '. It was first 

 described by Schneider (1898). 



The cross-section through the float- wall given by Chun (1902, fig. 79) correctly shows the relative 

 thicknesses of the codon and saccus, and their general structure. The figure omits certain features, the 

 chief of which are : the cuticle, the pneumatocyst, the nerve-plexus in the codon-ectoderm, the muscle 

 fibres of the codon-endoderm and of the saccus-ectoderm. These omissions have been corrected in 

 Text-fig. 3 accompanying this text. The ectodermal fibres run along the length of the float, parallel 

 to the crest. The endodermal fibres run round the float, in a circular direction. Thus in Text-fig. 3, 

 which represents a section at right angles to the longitudinal axis, only the cut ends of the ectodermal 

 fibres (m.ec) are shown, while the endodermal fibres (m.eri) appear in side view. The septa which 

 divide up the roof of the codon into pockets (Text-fig. 2) are folds of endodermal tissue and mesogloea 

 drawn out from the body-wall. The inflatable processes of the saccus, described on page 373, fit into 

 these pockets. As the animal grows, a regular subdivision of the pockets proceeds, new septa appearing 

 between those already formed. 



