HISTOLOGY: DIGESTIVE REGIONS 397 



coil. The distance between tentacles and electrodes was selected arbitrarily, but was kept as constant 

 as possible throughout the experiments. The results may be summarized as follows : 



(i) Nematocysts from large and small tentacles, from all animals tested, discharged at approximately the same 

 threshold. 



(2) The threshold appeared to be unaffected by the state of contraction in the tentacle. 



(3) Nematocysts from the basal growing region of the tentacles required strong shocks to discharge, or were 

 undischargeable. 



(4) Weak solutions of chloretone, chloral hydrate and magnesium sulphate all inhibited discharge partially or 

 completely, depending on their concentration. Solutions too weak to produce total anaesthesia produced definite 

 levels of anaesthesia, which were quickly reached and were maintained with little perceptible change over periods of 

 hours. 



(5) Strong solutions of many chemical substances, including anaesthetics, had an irritant effect, and caused 

 discharge, along with contraction of the tentacles. 



(6) Elevation and depression of the temperature caused depression and elevation respectively of the threshold. 



(7) Following anaesthesia, the normal discharge level was restored when the tentacle was transferred to fresh 

 seawater. 



(8) Segregated nematocysts (teased on a slide) failed to discharge in the electrical field. 



In all these experiments seawater controls were maintained. The method was too rough and ready 

 to be used for absolute determinations, but for comparative studies of the type carried out it gave 

 consistent results. 



The reaction to anaesthesia and to cold resembles that of muscle and, as the cnidoblasts contain 

 fibrils similar in appearance to muscle fibres, Parker and Van Alstyne's suggestion that it is these 

 fibrils whose reactivity determines the discharge level is a reasonable one (see, however, p. 395, para. 2). 



The high threshold of discharge in the basal regions of the tentacles indicates that the nematocysts 

 or their cnidoblasts are immature here. Histological examination confirms this. The immature region 

 extends a few centimetres below the tip of the ampulla in large tentacles ; exact data were not obtained. 



It is interesting to consider the case of the fish Nomeus which lives in commensal association with 

 Physalia and appears to escape injury from the nematocysts. The author has not observed Nomeus, 

 and the records do not clearly show whether the fish survives because it avoids being stung, or because, 

 when stung, it is immune to the poison. To the author it seems likely that the fish avoids being stung 

 and has innate or acquired behaviour patterns directed to this end. The situation in which it lives is 

 not really so hazardous as it might appear. The tentacles are only dangerous if contact is made with 

 them. No searching or exploratory movements of the sort performed by the tentacles of Velella take 

 place. To escape injury, all that a fish will require is small size, agility and an avoidance reaction.* 



The toxic and distressing nature of the sting to human beings has been the subject of frequent 

 comment (see particularly Phisalix, 1922). In the present study, the matter was not deliberately 

 investigated, although the author was frequently stung. The pain and shock produced are evidently 

 strictly proportional to the number of nematocysts discharged. Probably several hundred need to 

 penetrate the skin to produce really severe discomfort in a human.f 



8. Histology of the digestive regions 

 Our knowledge of digestion in the siphonophores is rather scanty. Most of the information available 

 is reviewed by Yonge (1930, 1 931) in discussions of digestion in the Coelenterata generally. In those 

 species which have been most thoroughly investigated, the evidence shows that a preliminary 

 extracellular digestion, of proteins only, takes place in the enteron, this being followed by intracellular 



* See Totton, Part I, p. 309. 



t Investigation of the Loggerhead turtle, which feeds on Physalia, reveals no immune boches in the blood (Dodge and 

 Lane, i960. Nature 185, pp. 330-331). Probably the nematocysts cannot penetrate the reptile's thick hide. 



DTM 

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