HISTOLOGY: MESOGLOEA 385 



The arrangement of the nervous system in the asexual medusoids (nectophores) of Physalia has not 

 been ascertained. The epithelia are hard to peel off for examination as strip preparations. No well- 

 fixed examples of larger medusoids were available for study, and sections of specimens fixed in museum 

 formalin have revealed nothing definite. 



Certain regions of Physalia show no trace of nervous tissue. Intensive studies have been made, 

 prompted by a feeling that nerves ' ought ' to be present throughout the animal, but the only result 

 has been to fortify the author's opinion that they are frequently absent. One of these regions is the 

 saccus-ectoderm, a tissue extending over an area of several square inches in medium-sized specimens. 

 As noted earlier, it appears to be inert, and incapable of active movements. It is so thin that nerves, 

 if present, would stand out clearly. Indeed, if nerves were present, this would be an ideal situation in 

 which to study their fine structure. In the codon-ectoderm, which is much thicker, and consequently 

 more difficult to treat, nerves can nevertheless be identified by a variety of staining methods. Thus 

 their absence from the saccus is all the more striking. It is possible to imagine a situation where very 

 long, fine nerve processes, sparsely distributed, proceed across the saccus from cell-bodies located in 

 the codon, but the improbability of such an arrangement will be apparent to anyone familiar with the 

 histology of the coelenterates. 



Nerves have not been found in the endoderm of any part of Physalia. It is possible, but not likely, 

 that they could have escaped observation in the gastrozooids, palpons and tentacles, for these regions 

 (particularly the last) are hard to examine by the strip technique. But in the endoderm of the codon 

 and saccus, where conditions for observing nerves would be nearly ideal, their absence can be vouched 

 for. It is worth noting that only rarely have students of the Hydrozoa found clear evidence for the 

 existence of endodermal nerves. They seem to be present in Hydra (Semal, 19526 and earlier workers), 

 but this is something of an exception. 



5. The MESOGLOEA 



The mesogloea in most regions of Physalia functions as a Stutzlamelle, that is, as a support for the 

 muscle fibres. When the fibres contract the mesogloea falls into folds ; when they lengthen the folds 

 are smoothed out (though they may not completely disappear). The mesogloea is noticeably thickest 

 where the musculature is most powerful (that is, in the tentacles and codon), and in such regions 

 permanent folds develop. It was at one time held that this folding of the mesogloea in coelenterates 

 was simply and solely the result of muscle activity in regions of high fibre density but Krasihska (1914) 

 argues convincingly against this view. 



In the float, tentacles, and gastrozooids, and in the stems of the gonodendra, strands of cytoplasm 

 stretch across the mesogloea from endoderm to ectoderm. In the codon and tentacles (where the 

 mesogloea is thick) these cytoplasmic bridges are often nucleated, and consist of complete cells (bridge 

 cells). Some are shown in Text-fig. 3 (br) and PI. XXVII, fig. 4. In older specimens the bridges are 

 thick, and contain several nuclei. Cell boundaries are hard to distinguish here, and the bridges often 

 look syncytial ; polyploid cells are common in them. The cytoplasmic bridges crossing the mesogloea 

 elsewhere are not nucleated. 



The bridges probably originate from the endoderm. Their exact limits are hard to define, as they 

 emerge from among a mass of muscle fibres on one side of the mesogloea and disappear into another 

 mass of muscle fibres on the other. Of all structures tending to obscure cellular relationships, muscle 

 is the chief offender. A sufficient number of cases has been observed, however, where cytoplasmic 

 bridges seem to arise directly from the endoderm, for the arrangement to be depicted as in Text-fig. 3. 

 Chun (1902) was also of the opinion that the bridges were endodermal. 



The bridges are often drawn out into long, thin strands crossing the mesogloea at very acute angles 



