HISTOLOGY: CUTICLE AND PNEUMATOCYST 389 



test where a whole saccus was used, all the tissue dissolved, except for the pneumatocyst. Subsequent 

 treatment with iodine and then with sulphuric acid gave results consistent with the other evidence 

 that the material is chitinous. 



A further investigation of the chemical nature of the cuticular material of the float has been carried 

 out by Dr K. M. Rudall, using the X-ray diffraction technique. A characteristic chitin pattern was 

 obtained. The pattern could not be defined as typically a- or typically /?-chitin. In addition to the 

 chitin, another substance soluble in benzene was found to be present ; it is probably lipid in nature. 

 No other coelenterate cuticle has yet been shown to contain lipid material. No trace of it was found 

 in Velella by Rudall (1955) using the X-ray diffraction technique. 



It is too early to say much on the significance of the lipid-like material, but if we are looking for 

 some substance which would provide the cuticle with a capacity to resist diffusion (chitin alone being 

 useless from this point of view) then a lipid would certainly meet our needs. In Velella, where the 

 stigmata, float-chambers and tracheae are believed to serve as a route for gaseous exchange (Chun, 

 1888), diffusion taking place directly through the walls of the tracheae, it would be surprising indeed 

 to find lipids associated with the chitin, for what is needed here is permeability to gases rather than 

 impermeability. Thus the presence of the lipid in the one case and its absence in the other accords 

 well with the presumed functional requirements. 



With regard to the secretion of the cuticular substances, the evidence is very circumstantial and 

 incomplete. Gland-cells occur in the ectoderm, and their distribution corresponds to a large extent 

 with the distribution of the cuticle. The gland-cells show many features in common with gland-cells 

 which, in other hydrozoans, are believed to secrete the cuticle (see, for instance, Berrill on Obelia). 

 However, it is a regrettable fact that none of the author's preparations simultaneously show both 

 distinct and recognizable gland-cells and an intact, unmistakable cuticle. The bulbous swellings at 

 the bases of the cuticular funnels (PI. XXVII, fig. 5) probably represent the sites of gland-cells, and 

 the tattered fragments of material overlying the ectodermal gland-cells in sections of a palpon 

 (PL XXVII, fig. 6) probably represent the cuticle, but in neither case are the relationships well- 

 enough shown to be regarded as conclusive. The gland-cells might have another function, or only 

 a proportion of them might be concerned in cuticle secretion, or some might secrete chitin and others 

 lipid. We do not yet know. A thorough cytological and cytochemical examination is needed. 



Ectodermal gland-cells occur in the codon, tentacles, ampullae, gastrozooids, palpons (where they 

 are very abundant) and stems of the gonodendra. Steche (1907), referred to similar gland-cells in the 

 palpons of Rhizophysa, and figured one (Taf. xi, fig. 4). He stated that they are also present in 

 Physalia but did not say where ; they do not appear in his drawings of the medusoids of Physalia, but 

 this is in accord with the present finding that gland-cells are absent from these members, and from the 

 jelly-polyps. The cuticle is also absent from these regions, as far as we can tell. 



The absence of gland-cells from the ectoderm of the saccus is puzzling, when it is considered that 

 the cuticle here (pneumatocyst) is fairly thick. The author's tentative explanation is that like the 

 cuticle elsewhere in the animal, the pneumatocyst is secreted by gland-cells, but that secretion stops at 

 a fairly early stage; the sites of the extinct gland cells would be shown by the knob-like thickenings 

 referred to above. Possibly, by virtue of its completely protected situation inside the air bladder, the 

 pneumtocyst is not subject to erosion or damage, and a continuous supply of the material is not needed ; 

 the exposed parts of the cuticle on the other hand, would need replenishment from time to time. If 

 this is true, the pneumatocyst would keep pace with growth of the float not by uptake of new material 

 but by stretching to cover a wider area. 



Although a detailed study of the gland-cells and their secretion (or secretions) has not been under- 

 taken, certain information is nevertheless available. The gland-cells give a strong P.A.S. reaction 



