386 DISCOVERY REPORTS 



(down to io°). The oblique orientation of the bridges is constant over wide areas, and may, in part, 

 represent tensions in the mesogloea set up by contraction in the two muscle sheets. 



In the tentacles and codon (and possibly elsewhere), the endodermal bridges have little blind pro- 

 jections which pierce the mesogloea on either side. In the tentacles these projections form a cluster 

 around each bridge-cell where it leaves the endoderm. In the codon the projections occur along the 

 whole length of the bridge-cell. It is not known whether these outgrowths have pseudopodial proper- 

 ties, but they look like pseudopodia. As to their function, that is a matter for speculation. No amoe- 

 bocytes have been observed and the bridge-cells may fulfil the function of transporting nutritive 

 substances across the mesogloea; their outgrowths could serve to distribute the substances to the 

 mesogloea. On the other hand they might serve to collect metabolic waste, like the outgrowths from 

 the flame-cells which penetrate the parenchyma of planarians. It is not intended to suggest that the 

 bridge-cells are homologous with flame-cells; on the contrary, the presence of 'pseudopodial' out- 

 growths may indicate that they represent amoebocytes which have become fixed in permanent 

 positions. 



Large pieces of mesogloea, freed from muscle-fibres, can be prepared from the codon and saccus by careful 

 dissection. The tissue is first briefly immersed in gentian violet; then it is placed in a petri-dish of distilled water 

 under a low-power binocular microscope and the muscle fibres are stripped away with watch-maker's forceps. The 

 sheet of mesogloea (pure, except for the bridge-cells) is then washed in 70% alcohol to remove the stain. It can 

 be restained in any way desired. In the present investigation, iron haematoxylin was found to show up the fine 

 structure well, if differentiation was not carried too far. 



Study of stained sheets of mesogloea, both by teasing in glycerine and by examination intact at high 

 magnifications, reveals that the mesogloea has a laminated structure. The laminae are broad, flat 

 sheets which cross one another at a variety of oblique angles, without any apparent regularity in 

 orientation. The mesogloeal substance is finely fibrillated. This, and the laminated structure, can be 

 seen in PL XXVII, fig. 3. The laminae do not appear to be regularly interwoven. In Calliactis and other 

 actinians, on the other hand, the mesogloea has a lattice-like structure of ' undulating sheets ' as in a 

 woven fabric (Chapman, 1953), and the warp-and-weft arrangement is regular in character, with a 

 definite orientation in particular regions. It is thus more highly organized than the mesogloea of the 

 float in Physalia. 



The periodic acid-Schiff technique has been applied to sections of Physalia material. The mesogloea 

 in all regions gives a strong positive reaction (indicating abundant polysaccharide), particularly near the 

 muscle interface. This can be seen in the transverse section of a palpon (PI. XXVII, fig. 6), where the 

 mesogloea forming the central core of the villus has a definite, dark outer ring. X-ray diffraction 

 analysis of strips of mesogloea (prepared by the method described above) was carried out by 

 Dr K. M. Rudall. Collagen, as recognized by its wide-angle diffraction pattern was found in abun- 

 dance in the thick mesogloea of the codon. It was also found, but less abundantly in the mesogloea of 

 the saccus. For a survey of the distribution of collagen and chitin in coelenterates, Rudall (1955) 

 should be consulted. 



The mesogloea in the nectophores and jelly-polyps (' Gallertpolypoide ') is of the gelatinous, dis- 

 tended type characteristic of medusae and medusoid members. Such mesogloea seems to serve 

 primarily for buoyancy (Jacobs, 1937). In many medusae it is fortified by a system of orientated fibrils 

 (Alvarado, I932) and this is true of the swimming-bells in siphonophores such as Hippopodius (Mackie, 

 unpublished). However, the nectophores and jelly-polypoids of Physalia have not been investigated 

 from this point of view. The functional significance of the jelly-polypoids remains problematical. One 

 is at a loss to know whether they serve for flotation or food storage or whether indeed they have a 

 function at all, or are merely vestigial. 



