90 AMERICAN JOURNAL 



including the Ascidians, and that the former are, as he expresses 

 it, " true articulates and not niollusks, and that their proper 

 place is among the worms, forming a group near the tubicolous 

 Annelids," 



A statement at once so startling and so utterly at* variance 

 with the received ideas upon the subject, naturally awakens a 

 sentiment of opposition ; such being the case, we should en- 

 deavor to use the utmost impartiality in discussion, and to give 

 due consideration to the facts bearing upon either side of the 

 question. It is, I believe, generally admitted that, in classify- 

 ing forms which lie upon the boundaries of the great subdivisions 

 so closely as to render their position a matter of doubt, a con- 

 clusion is more fairly reached by considering the sum total of 

 the characters than by an exclusive classification based upon 

 any one of them; and also that in arriving at a decision a com- 

 prehensive view of the general types of structure is more re- 

 liable than minor morphological considerations. Takiilg this 

 view of the case, we may judge more clearly of the value of Mr. 

 Morse's proposition by comparing the general structure of the 

 Mollusca with that of the Annelids and then deciding from the 

 facts, to which of them the structure of the Brachiopods and 

 Polyzoa offers the greatest affinity. 



Clark has shown very clearly* that the mollusca as a group 

 are characterized especially by a bilateral development of single 

 organs, that is, " among the zoophytes and articulates we find 

 an excess of repetitions, of multiplicity of parts ; but here 

 (among the mollusca) every part or organ is single or in a single 

 pair, and, instead of repeating itself, it develops its uniformity 

 to relatively extreme proportions." 



The main characters of the mollusca are as follows : 



The circulatory system varies in extent, the ascidians possess- 

 ing a simple unilocular contractile vesicle and incomplete vascu- 

 lar apparatus, while the cephalopod developes a trilocular heart 

 with a distinct arterial and venous system. The heart, when it 

 exists, is usually on the opposite side of the alimentary tube, 

 from the principal nervous centre. The blood is colorless or 

 colored. t 



The nervous system essentially consists of a collar of nerves 

 and ganglia around the oval opening or anterior portion of the 

 alimentary tube. The principal nerve-mass is usually on the 

 opposite side from the heart. 



The alimentary system consists essentially of a tube invariably 



* Mind in Nature, 1865, p. 195. 



t Colored blood has been observed in Chiton; in Tunicata by Milne 

 Edwards ; the circulating fluid oi Hinnites is of a deep wine-color. 



