HISTOLOGY: DIGESTIVE ORGANS 399 



jections of the endoderm into the enteron known as 'villi', which have often been described in the 

 earlier literature (for example, Huxley, 1859). In young gastrozooids the basal region is longer than 

 the buccal, but in mature examples the two regions are approximately the same length when the zooid 

 is relaxed. The gastrozooid has considerable powers of expansion and contraction, tending to 

 elongate during exploratory activity. 



The buccal region (Text-fig. 5) in a relaxed zooid shows a regular folding of the endoderm into 

 longitudinal ridges. These ridges are visible to the naked eye in ordinary preserved material. In 

 zooids where the buccal region was fixed in an expanded state, the ridges are inconspicuous or absent, 

 and it may be concluded that the folding is a mechanical device making possible the accommodation of 

 the bulky endoderm cells when contraction takes place. 



The cells of the buccal region are secretory. The secretory matter fills the distal part of the cells and 

 takes the form of rounded droplets, or, in some cases, of a finer, more diffuse substance. The droplets 

 may measure 10// in diameter. They are well seen in unstained F.W.A. material, where they show an 

 amber coloration. In gelatin sections of Ca-formaldehyde material, stained in Sudan black, the 

 droplets show little if any darkening, and can therefore be said to contain little lipid material. They 

 stain strongly with eosin in Zenker material, and with iron haematoxylin in Helly. The P.A.S. 

 reaction is strong in the region of the secretion, but the secretion droplets do not stain any more 

 strongly than the matter surrounding them. Protein tests have not been carried out, but are clearly 

 indicated for any future study, in view of the probably enzymatic character of the secretion. Where 

 the secretion is more diffuse, we may be dealing with cells in the process of elaborating their product, 

 this not yet having coalesced to form the droplets. Alternatively, the cells with the diffuse secretion 

 might constitute a distinct type, like the 'cellules spumeuses' in Hydra, while the cells with the droplets 

 would correspond to the 'cellules sphe'ruleuses' (Semal-van Gansen, 19546). A further investigation 

 along the lines of Semal-van Gansen's beautiful study is required. 



In gastrozooids fixed when not feeding, the buccal cells are packed with secretory matter. In one 

 specimen, fixed while actually attached to a captured fish, many of the cells appear empty, although it 

 is difficult to compare the appearance of cells in the expanded, flattened specimens and in the folded, 

 contracted ones. If correct, however, this observation would indicate that the buccal cells tend to 

 discharge their product during feeding, and it would seem likely therefore that the secretory matter 

 consists of the enzymes responsible for the preliminary extracellular digestion. It was mentioned 

 above (page 390) that a glutinous substance produced by captive specimens of Physalia seems to 

 originate from the gastrozooids, and the author is inclined to suppose that this substance may derive 

 from the buccal gland cells by degenerative breakdown in conditions of oxygen shortage. However, 

 Mettey and Hamon (1949) hold that the buccal gland-cells in the gastrozooid of Abylopsis secrete a 

 viscous substance as their normal product, and that it serves for the agglutination of prey. 



In Hydra, the cells concerned in the production of proteolytic enzymes undergo a series of secretory 

 cycles, discharging their product when food is ingested, and resecreting it again rapidly, until they 

 reach a stage of senescence, when they quit the endoderm and drift out into the enteron. It is too early 

 to say whether, in Physalia, the buccal cells undergo cycles of secretion or, if they do, how many 

 cycles can take place, but there is evidence that cells or large portions of cytoplasm containing secre- 

 tion-droplets are shed from the endoderm and mix with the food matter undergoing digestion. Such 

 objects, sometimes apparently consisting of whole buccal endoderm cells, have been found free in the 

 enteric fluid. Loose droplets are also found, and these sometimes appear to have fused together into 

 large globules, or to have formed aggregates. Parallels to these observations are recorded by Semal- 

 van Gansen for Hydra. 



In Hydra the gland-cells usually have two flagella projecting into the enteron from their free edge. 



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