164 MORSE, CLASSIFICATION OF MOLLUSCA 



Objecting as all must to the introduction of a new 

 name, still one so appropriate as that proposed by Mr. 

 Hyatt, in lieu of one that has no relation to the Branch, 

 except its traditional use, is certainly worthy of considera- 

 tion, as it so clearly indicates what is believed to be the 

 fundamental idea in the Branch, that of the Sac. 



It might be said, in one sense of the word, that all ani- 

 mals are bags, or sacs, in various degrees of development. 

 And if we mistake not, Prof. Pierce of Harvard Univer- 

 sity has expressed this idea, modified by saying that one 

 is a radiate sac, another a simple sac, another an articulate 

 sac, and finally a vertebrate sac, or a sac having two com- 

 partments. Viewing the Radiates as degradational, in re- 

 lation to the higher animals, or partaking a plant-like charac- 

 ter, we may justly be allowed to remark, that the Mollusks, 

 as a type, present the sac feature most completely, for 

 nowhere (with few exceptions, e. g. Cirripeds), do we find 

 the various organs so essentially concealed, or possessing 

 the power of retraction within a sac, as in the Mollusca. 

 And that this is the leading feature in Mollusca might 

 properly be inferred from the following ; that in the four 

 prominent branches of the Animal Kingdom, we have 

 sketched out, in the incipient stages of the embryo, or at 

 least, in its first indications of permanent characters, its 

 typical features. 



Thus, in the vertebrate ovum, after segmentation, we 

 have the area pellucida, and primitive trace as indicating 

 the future region, and direction of the vertebrate column. 

 Among the Articulates, we have the transverse division of 

 the embryo : and certainly the most prominent feature in 

 the Molluscan embryo is the sac or mantle ; as in the Gas- 

 teropods, where we not only have in the embryo a mantle 

 developed, but a distinct nautoloid shell, from which the 

 little animal thrusts himself. In Cephalopods also, as K61- 

 liker has shown in the development of Sepia officinalis, the 

 mantle, or sac, is the first figure traced on the germ 

 mass. 



In my search after homologies between the different 

 groups in this Branch, I always met with difficulty in the 

 relations of the classes ; — and though many of the views to 

 be presented, I had long ago worked out, and had consid- 



