HISTOLOGY: DIGESTIVE ORGANS 401 



pigment found in the enteron may have been excreted in this way; the aggregates of pigment, how- 

 ever, are clearly derived directly from the food, and are sometimes surrounded by tissues from the 

 fish. The pigment found in the endoderm of Hydra is also probably melanin derived directly from the 

 food, and in this case originating in the eyes of the Daphnia on which Hydra preys (Semal-van Gansen, 

 19546). It was earlier supposed to derive from the chromatin of broken-down nuclei (Schlottke, 1930). 



The occurrence of nematocysts in the cells of the endoderm and in the enteric contents is at first 

 rather surprising, although a similar finding has been recorded for Halistemma (Claus, 1878). The 

 nematocysts are clearly undergoing digestion, for they have no surrounding cnidoblasts. In the villi 

 irregular, amber-coloured lumps are sometimes found which probably represent nematocysts in 

 advanced stages of digestion, but more often intact, spherical nematocysts are seen, and these may lie 

 in vacuoles situated well down towards the base of the cells. 



It might be supposed that these nematocysts are senile ones which have been removed from the 

 ectoderm, crossed the mesogloea to the endoderm and have then passed to the gastrozooids for 

 digestion. If this were so, one would expect to find nematocysts in stages of migration across the 

 mesogloea. However, study of numerous sections from various parts of many specimens has revealed 

 no evidence, direct or indirect, that this takes place. The nematocysts in question are the large and 

 small isorhizas characteristic of the tentacles. What probably happens is that the gastrozooids apply 

 themselves to the captured fish while some of the tentacles are still attached to it, and tentacular 

 matter is ingested together with the tissues of the fish. Only in this way does it seem possible 

 to account for the presence of undischarged capsules in the enteric contents, and, later on, the villi. 

 It is interesting to note that the cells of the villi can engulf objects such as the largest nematocysts, 

 whose diameter is typically about 25 ju. 



In the general endoderm of the basal region of the gastrozooid (that is, in the parts lying between the 

 villi) three main cell types are distinguishable. The first type, which is in a majority, is apparently 

 non-glandular, and is probably simply absorptive. Cells of the second type are closely similar to the 

 buccal gland-cells, and may even be identical to them. Cells of the third type are evidently glandular, 

 but they differ from the cells of the buccal region. They resemble more the 'cellules glandulaires 

 moyennes' which Mettey and Hamon (1949) described in Abylopsis. In the latter form, these cells 

 have a region of very basophil, sudanophil cytoplasm ('chromatoplasm') around the nucleus: they 

 are believed to produce a diastase. In Physalia the cells thought to be comparable have a similar 

 region of basophil cytoplasm around the nucleus; this area also stains strongly in P.A.S. preparations. 

 The cytology of these regions has not yet been investigated in detail, and no further observations can 

 be added. 



The palpons, as stated above, probably do not play an important role in digestion while the 

 gonodendra are still attached to the parent colony; indeed, the younger animals manage without them. 

 In the structure of their endoderm they resemble young gastrozooids. The villi are well developed, 

 but the buccal region lacks the heavy concentration of glandular cells found in the gastrozooids and, 

 probably as a result of this, is not thrown into folds. In the distal extremity, where the mouth 

 ' should ' be, an indentation can sometimes be seen, but sections have not revealed an actual opening 

 here. If tightly constricted however, a mouth or incipient mouth, might not be easy to detect.* 



The villi in a number of cases have been found to contain food vacuoles, with nematocysts and 

 pigment matter in them, but palpons have not been observed feeding, and it seems unlikely that they 

 do feed at this stage; the food matter could reach them from the general enteron, having been taken 

 in via the gastrozooids in the first place. Bits of semi-digested food and pigment have been found 

 elsewhere in the enteron, for instance in such remote regions as the ampulla of a large tentacle. 

 * Totton (Part I, p. 354 and Plate XXV, figs. 2, 3) shows that openings do exist. 



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