of the Powers of producing Heat and Light. 245 



ducing heat, or a tendency to it, which at length attains its 

 maximum, whence we began the survey. 



There appears to be no instance in which all the species, 

 either of a great or of a subordinate group, are luminous, con- 

 trary to what obtains with respect to animal heat ; nor have 

 all the species this character in either of the two osculant 

 groups certain members of which are luminous. On review- 

 ing the entire animal kingdom with respect to the compara- 

 tive number of species at present known or regarded to pro- 

 duce heat and light respectively, the facts appear to be as 

 follows. In the vertebrated animals animal heat is found in 

 all the species composing the subordinate groups of Mammalia 

 and Birds, and in a low degree in a ievf species of Fish. 

 Its existence in Reptiles and Amphibia has not yet been 

 shown. In the osculant Annelida a few species are luminous. 

 In the Annulose circle, a few species among the Ametabola, 

 Mandibulata, and Haustellata are luminous; and a few others, 

 of the second of these groups, have, it is probable, but not yet 

 certain, a low degree of animal heat ; while of the Crustacea 

 very many species are luminous. Among the Radiata many 

 species of Medusida have this property; among the Acrita 

 certain species oi Polypi Natantes ; the osculant Tunicata pre- 

 sent many of the most extraordinary instances of luminous 

 power ; and a few species also in the succeeding great group 

 or subkingdom of MoUusca, by which, through the Cephalo- 

 poda, we arrive again at the Vertebrata, have a degree of the 

 same power. 



Thus animal heat exists principally among the typical Ver- 

 tebrata, — Mammalia and Birds ; and animal light principally 

 among those animals of the other subkingdoms of zoology the 

 structure of which is the furthest removed from the verte- 

 brated type : and there are various indications, between the 

 Crustacea and the Mammalia, on reviewing all the phaeno- 

 mena presented by the intervening subordinate groups of 

 Fishes, Annelida, Ametabola, Mandibulata, Haustellata, and 

 Arachnida, of the reciprocal substitution of a degree of the one 

 power for the other, as we pursue the facts showing approxi- 

 mately the diminution of animal light upwards in the scale of 

 organization, through the above series, from the Crustacea to 

 the Mammalia, and the diminution of animal heat downwards 

 in the scale, from the Mammalia to the Crustacea. The facts 

 affording these indications will be detailed and discussed in 

 the Memoir. 



Although Mr. Macleay's classification has been adopted in 

 this notice as being at once the most convenient for the pre- 

 sent purpose, and the first approximation yet effected to the 



