HISTOLOGY: NERVOUS SYSTEM 383 



successfully on a variety of coelenterate material, was used to stain paraffin sections and whole strips 

 of tissue taken from Physalia. Results have been only partially successful. In many preparations, the 

 nerves show up quite well, but there is generally too much background staining (particularly in the 

 region of the muscle-mesogloea interface) for the nerves to stand out in fine detail. This fault could 

 probably be rectified by further trial and error adjustment of the staining schedule. In paraffin 

 sections, nerves have not been positively identified, partly because the formaldehyde-sublimate 

 fixative recommended for the sharpest nerve staining is rough in its action on the tissue generally, 

 and partly because the imbedding technique further distorts the tissue relationships, where much 

 muscle and collagenous mesogloea are present. Nerves have been found only in the strip preparations. 



Chun's finding of nerves in the ectoderm of gastrozooids has been confirmed. A continuous ecto- 

 dermal plexus extends throughout gastrozooids, codon, palpons and gonophores. In PI. XXVII, fig. 2, 

 a portion of the nerve-plexus from the codon-ectoderm is shown. A tripolar neuron (centre) and a bipolar 

 (lower right) can be seen. The dark bands crossing the field from top to bottom are the longitudinal 

 muscle-ridges of the ectoderm. In the codon-ectoderm there were estimated to be about 140 nerve- 

 cells per sq. mm. In the gastrozooids, there appear to be about half this number. Tripolar neurons 

 predominate ; out of fifty studied in a particular area, thirty-two were probably tripolar, the remainder 

 being bipolar. A few multipolar cells (4- and 5 -polar) were also seen. 



Bi-nucleate neurons also occur sparsely in the float. This is not a unique finding; Hertwig and 

 Hertwig (1878 a) found bi-nucleate neurons in Cunina. Nervous tissue in coelenterates is evidently 

 post-mitotic in the main but, where bi-nucleate neurons occur, it may be that mitosis has taken place 

 in the nucleus of a young neuron after differentiation has proceeded too far for the cytoplasm also to 

 divide. 



The exact character of the inter-neural associations has not been determined. It is often difficult 

 to decide where a fibre from one nerve ends and another begins, but the preparations are not 

 stained sufficiently precisely for accurate observations on the fine structure to be possible. Fibres have 

 been found to run for distances of up to 100// without branching or associating with processes from 

 other nerves. 



In no region examined was there any hint of an elaboration of the plexus into orientated tracts. 

 Pieces of tissue were examined from several regions of the float, and it is unlikely that tracts, such 

 as those which occur in the stem-bladder of Physophora (Korotneff, 1884, Taf. 15, fig. 26) would have 

 escaped observation. The plexus is distributed evenly in all regions examined. 



In some of the preparations, sense-hairs (in groups of two or more) can be seen emerging from the 

 surface of the ectoderm (Text-fig. 3). In most such cases, a nerve-cell (s.n) can be found in the region 

 underlying these sense-hairs. In a few cases a conical projection of the nerve-cell can be seen passing 

 up to the superficial region, the sense-hairs being embedded in it. Around the roots of the sense-hairs, 

 the cytoplasm of the conical projections shows a dense aggregation of very fine granules. Thus, 

 although the evidence is incomplete, it is probably true to say that the sense-hairs belong to the nerve- 

 cells, not to the surrounding muscle-cells, and that the majority of the nerve-cells possess sense-hairs. 

 Where one or other component is apparently absent, this can usually be attributed to vagaries in the 

 staining technique. It was found that some gastrozooids fixed in Ca-formaldehyde showed the sense- 

 hairs and their relationships with the nerve-cells quite clearly, when Peter's glycine developer 

 (Peters, 1955) was used. It is not quite clear whether the sense-hairs penetrate the cuticle, but it 

 would seem reasonable to suppose that they do. 



Attempts to demonstrate nerves in the tentacles have so far been unsuccessful. It is highly probable 

 that nerves do exist here, particularly in view of Parker's physiological findings (Parker, 1932). The 

 exceptionally muscular nature of the tentacles makes them very difficult to examine histologically. 



