250 LECTURE XIX. 



then have gained its instruments for leaping, as the caterpillar ac- 

 quires its organs of flight, and the concomitant development and 

 metamorphoses of the organs of sense, of digestion, and of generation 

 would have been closely analogous in both animals. 



LECTURE XIX. 



ARACHNIDA. 



There remains one class of articulate animals to be considered in 

 our present ascending survey of the animal kingdom, and which, 

 therefore, you will conclude to be the highest organised of the 

 homogangliate Invertebrata. Yet the species which are grouped 

 together under the name Arachnida never acquire wings : many are 

 parasitic, many terrestrial, and a few are aquatic ; there are some, 

 however, that, notwithstanding their apterous condition, can rise and 

 float through the air, which they effect in a manner analogous to our 

 balloon aeronauts, by manufacturing and suspending themselves to a 

 foreign substance light enough to be buoyed up and wafted along the 

 atmospheric currents. The animals to which I allude, and whose 

 anatomy and physiology I propose to make the subject of the present 

 lecture, are those commonly known under the name of mites, scor- 

 pions, and spiders. 



You will be disposed to ask why these Articulata are held superior 

 to insects ? They present a more concentrated form of the nervous 

 system and of the heart : the larger species, likewise, present a 

 higher condition of the respiratory system, which is less diff'used 

 than in insects, and in some consists only of air-sacs or lungs. Per- 

 haps the most essential mark of the superiority of the Arachnida 

 is the course of their development. The spider undergoes no me- 

 tamorphoses comparable with those of insects. It is at no period of 

 its development an apodal worm. The mature form is sketched out 

 from the beginning, the divisions of the body characteristic of the 

 perfect animal are established before the vitelline mass is included by 

 the tegument ; and, long before the characteristic palpi and legs are 

 completed, the equally characteristic ocelli are developed upon the 

 head. If you should still be disposed to think that the superior 

 organs of vision and the wings of insects are essential signs of a 

 higher organisation, by parity of reasoning, you must be prepared to 

 place birds above mammals. 



