DISTRIBUTION OF ORGANISMS. 157 



point on which the benefit of further investigation is desir- 

 able. In the case of the Arachnida (mites and spiders), the 

 highest articulate class, no humbler form is traceable in the 

 embryo ; it is therefore impossible to assign them any pedi- 

 gree. Can it be possible that the arachnida, or these with 

 the insecta, have sprung almost or wholly at once from inor- 

 ganic elements under the proper electric influences? On 

 this subject, we are quite unprepared to make any positive 

 affirmation ; but it certainly is remarkable that in no depart- 

 ment of the animal kingdom, besides the infusoria and 

 entozoa, have there been more frequent appearances of an 

 aboriginal commencement of life than in the insecta. The 

 acarus so often produced from certain solutions, where ova 

 were rigidly excluded, is a lowly member of the arachnida. 



We now come to the MOLLUSCA, a portion of the animal 

 kingdom, the importance of which, in point of numbers and 

 the part they play in creation, none but students of zoology 

 could fully appreciate. The infinite variety of bivalve and 

 univalve shells presented upon our own coasts and brought 

 from all parts of the world, will convey some idea of the mul- 

 titude of forms comprehended under this sub-kingdom. The 

 whole mass is after all resolvable into three divisions ; one of 

 them comprising headless mollusks in bivalve shells ; the other 

 two, headed mollusks in univalve shells (some, however, of 

 all the three divisions being naked). The whole sub-kingdom 

 appear to have a very brief genesis in the radiata, the only 

 preceding forms in embryo being the infusorial and polypian. 

 Here, too, as in the Articulata, we find that we must start at 

 a point very near the fountain-head of organic existence. 



In the headless division, naturalists place three sub-divi- 

 sions, called by them classes, in the following rank, according 

 to ascending grade of organization Tunicata, Brachiopoda, 

 and Lamellibranchiata. The two latter are the shell-fish of 

 popular observation, headless, and mostly sessile, or destined 

 to spend their lives in fixed positions. The Tunicata are 

 similar in all essential respects, except in being of humbler 

 organization, an inclosed, not in shells, but in a cartilaginous 

 or coriaceous integument ; whence their name. It thus ap- 



