HISTOLOGY: CUTICLE AND PNEUMATOCYST 3»7 



The cuticle and pneumatocyst 



The pneumatocyst is the lining of the saccus cavity. It can be regarded as an internal cuticle, for 

 it is secreted by invaginated ectoderm and remains continuous with the rest of the cuticle via the 

 apical pore, which is the site of invagination. It is a regular feature of the float in Cystonectae and 

 Physonectae. Usually, as in Physophora (Leloup, 1941), it is a thick conspicuous structure, evidently 

 having a skeletal function, whereas the external cuticle is either absent or so thin that it escapes observa- 

 tion. In Physalia the pneumatocyst (first described by Schneider, 1898) is a transparent membrane 

 about I // thick in a young specimen. The external cuticle is probably much thinner. 



It seems possible to distinguish two clear types of cuticle in the Hydrozoa. The first type of cuticle, 

 found in Hydra fusca (Schulze, 1871) and Cunina (Hertwig and Hertwig, 18786) is apparently not 

 secreted by specialized gland-cells. It is a very thin membrane attached to the ectoderm by means of 

 numerous small bulbous protuberances, many of which are found embedded in each ectodermal cell. 

 Such a cuticle is probably secreted by the whole ectoderm. In the second type, seen in Eucopella 

 (Lendenfeld, 1883), the cuticle is secreted by specialized gland cells distributed throughout the 

 ectoderm. This type of cuticle is often very thick and may be laid down in layers as the perisarc 

 or theca. The cuticle of Physalia is here considered to belong to the second type, for the evidence 

 strongly suggests that it is associated with special gland-cells. 



In dealing with cuticles of the second type, particularly where a perisarc is formed, it should be 

 noted that the concepts of the production of the perisarcal substance as a viscous secretion from 

 ectodermal gland cells containing refringent inclusions, its subsequent hardening, and its reabsorption 

 in budding regions are at least seventy years old, inherent in the writings of Weismann, von Lenden- 

 feld and other workers of that period. Berrill (1949), in his studies on Obelia, appears to have arrived 

 more or less independently at the same conclusions. The term ' polymerization ' is now used to describe 

 the process of hardening in the cuticular substance (Hammett, 1943). 



Weismann's work has been criticized by Berrill (1949, p. 235) on the grounds that his illustrations, 

 though beautiful and accurate 'were interpreted in an extremely static manner remote from any 

 concept of a living organism'. This may be true to some extent of the particular publication that 

 Berrill refers to, but in justice to Weismann we should note that he also provided us with a detailed 

 account (Weismann, 1 881) of active processes in living hydroids somewhat along the lines of Berrill's 

 own studies. Again, in the particular case where Berrill (p. 245) finds it necessary to interpret 'at face 

 value' some illustrations which seem to show that the perisarc can be laid down and dissolved 'under 

 certain more or less obscure conditions ', it would be more appropriate to consult Weismann's account 

 of the process in Plumidaria (Weismann, 1880). 



The distribution of the cuticle in Physalia has been investigated by teasing fixed material under the 

 dissecting microscope. The earlier workers, for instance, Schulze (1871), used maceration techniques 

 on fresh material. After fixation in Bouin, the cuticle can be lifted or scraped away with fine needles 

 and its distribution can thus be determined. By means of this technique the cuticle has been found to 

 extend over the float, gastrozooids, tentacles, ampullae, palpons and gonodendra. It has not been 

 found in the gonophores, nectophores or in the jelly-polyps, but in these regions, a very thin cuticle 

 might escape detection by the method used. For studying the fine structure of the cuticle, it was 

 found that certain silver preparations served the purpose. These preparations were some of the 

 numerous ' failures ' from attempts to stain the nervous system, and the exact conditions under which 

 the cuticle becomes impregnated have not been worked out. No other methods proved to be of any 

 use; the cuticle is normally transparent, and shows no particular affinity for acidic or basic coal-tar 

 dyes, carmine or haematoxylin. No greenish coloration was produced by treatment with chlorazol- 



