INVERTEBKATA, CRYPTOGAMIA, MICROSCOPY, ETC. 629 



crayfish affortls an extremely easy subject, and the author has been 

 enabled by its aid to convince himself that acids or alkalies are not 

 poisonous because of their acidity or alkalinity. This is shown by 

 the following facts : A crayfish can live for two or three hours in 

 water containing 25 grammes per litre of acetic acid ; if there are 

 only 20 grammes per litre it can live for half a day. The mineral 

 acids are the most fatal ; in 5 grammes (per litre of water) of sul- 

 phuric acid, a crayfish dies in less than an hour ; if the water only 

 contains 1 gramme per litre it may live for ten to twelve hours. 

 Nitric acid has still more marked effects ; half a gramme in a litre of 

 water will kill a crayfish in two or three hours, and if the quantity of 

 acid be doubled, the creature will die in half an hour at the outside. 

 The first tissue to be aff'ectcd appears to be the muscular, and it is a 

 long time before the effects are lost after the animal is removed from 

 the acidulated water. 



Alkaline solutions appear to have a more marked eflfect ; the least 

 hurtful is baryta, of which 3 grammes may be put into a litre of water 

 for the crayfish to remain alive for two or three hours. The most fatal 

 alkali is ammonia ; half a gramme in a litre of water has an almost 

 instantaneous effect, and even with one-tenth of a gramme the crayfish 

 dies in two or three hours. It is, in fine, even more fatal than 

 strychnine. 



The difierences in efiect would appear to be due to the difierent 

 degree in which the drugs are absorbed by the respiratory organs. 



Head of the Lobster.* — Professor Young shows some important 

 relations in the grooves on the carapace. The so-called " cephalic 

 groove " is really double : its anterior lateral branch starts from the 

 antennary sternum, and marks off a prestomial segment. This dis- 

 tinction of a prestomial region is paralleled in the Annelids, where 

 also it is supplied from the supra-cesophageal ganglion. In the Stoma- 

 poda and in the Poutellida^ the antennary segment is free. The posterior 

 lateral branch begins rather posteriorly — behind the maxillipedes — 

 and passes forward to join the former opposite the articulation of the 

 mandible. Fi-om the posterior position of this latter groove, which 

 marks tbe hinder limits of the strictly oral segments, it is seen that no 

 place is left for the sterna of the maxillipedes' segments, and their terga 

 are but small. 



Claus's statement that the mandibles do not originate from the 

 third pair of Nauplius limbs, but from the body-surface behind them, is 

 borne out by the observation of such a development of it from the 

 lateral oral margin behind and on the inner side of the third pair of 

 appendages in some Macrurous larvfx^ ; it is thus similar in origin to 

 the labrum ; probably the third larval pair of limbs fuses with it, con- 

 stituting the mandibular palp, which is supplied from the sujira- 

 CESophagcal ganglion, which also supplies all the prestomial region. 



This prestomial region includes the eyes, antenna;, antcnnulcs, 

 with the sense-sacs at thcii' base ; it is a primitive division of the 

 body, and its relation to the nervous centres supports the view that 



* ' Joum. Anat. et Pliysiol.' (Humphry), xiv. (ISSO) p. 348. 



