CONSIDERATIONS. 1 I 37 



Lastly : there exist some families, as Trachean Arachnida, 

 and the Chilognatha, where the stigmata do not communicate, 

 the trachea', ramifying directly from them to be spread over the 

 body.' 



The branches of the trachea* are in general slightly tapering, 

 as the arteries of higher animals ; but sometimes, especially in 

 the Lamellicron Coleoptera, e they form, from space to space, 

 vesicles of different sizes, from which small branches are sent 

 off to the neighbouring organs. 



It appears that ammoniacal gas is that which most quickly 

 destroys insects. In azote they can live several days ; and 

 though a Melolontha vulgaris was observed to fall motionless 

 when immersed in pure hydrogen for fifteen minutes, yet it 

 returned to life after remaining fifty hours in that gas. 



Respiration, it seems, can only be carried on by moistened 

 surfaces ; hence the lungs of the higher animals are always 

 moistened by their own transpiration, whilst aquatic animals 

 have often exterior branchice moistened by the water. Some 

 Annelida, as the earth-worm and leech, which breathe by the 

 skin, have this always moist : in the former, from the effect of 

 the damp earth, which they inhabit ; in the latter, by a viscous 

 matter, which covers the skin. But in this respect, the Onisci 

 offer the last degree of possibility, as they breathe air by means 

 of branchice ; yet, as the respiratory surfaces of their branchice 

 are not entirely exposed, but covered by lamina?, parts of the 

 organs themselves, it is possible that they may never be entirely 

 dry. 



In following the scale of gradation of the respiratory system 

 in the articulated animals, we find it almost disappear in the 

 Annelida abranchia, where the respiration is performed by the 

 whole surface of the body. By degrees we see it reappear, 

 either as branchice, lungs, or lastly, as trachece ; and this 

 difference depends, on the one hand, upon the medium which 

 these animals inhabit, and, on the other, upon the gradation 

 which the respiratory and circulatory systems follow. 



In the Crustacea these organs are external, at least are 



f This is the case also with some Lepidopterous larva?, in which I have ob- 

 served the trachea to ramify directly from the stigmata. This might naturally be 

 expected, when we consider that the progressive development of individuals re- 

 sembles that which we find as we trace the gradation of organs from one group to 

 another. — E. D. 



* And the Buprrst?da>. but not the Etaterider. — E. D. 

 NO. II. VOL. II. T 



