180 MORSE, CLASSIFICATION OF MOLLUSCA 



Explanation of Plate IV. 



Series I. Represents a typical figure of each principal 

 group in Mollusca — viz., P, Polyzoa ; B, Brachiopoda ; T, 

 Tunicata ; L, Lamellibranchiata ; G, Gasteropoda ; and C, 

 Cephalopoda — (M, indicating Monomyaria, of the second 

 series). These figures are represented anterior end down- 

 ward, the dorsal region being turned to the left. The tube 

 within each cut, represents the intestine, the larger end of 

 which is the mouth, and the smaller end the anus. The 

 harp-shaped figure represents the heart, and the star repre- 

 sents the pedal ganglion. 



Series II. Represents similar views, with less detail. 

 The dorsal region in this series is uppermost, and the an- 

 terior end, is turned to the left, as indicated by arrow A. 

 The curved line indicates the intestine, the large end be- 

 ing the mouth. 



Series III. Represents transverse sections of corres- 

 ponding figures in Series II. 



Series IV. Represents the mean position in nature, of 

 the antero-posterior axes of the figures represented above, 

 A, Anterior pole, P, Posterior pole. The vertical rows 

 of figures are identical. 



Note. Since lines 7 — 12, page 164, were printed I have had an op- 

 portunity of quoting the remarks made by Professor Peirce as reported 

 in the Proc. Amer. Acad. Arts and Sci., Vol. Ill, p. 8. 



"Professor Peirce also presented a communication upon the form as- 

 sumed by an elastic sac containing a fluid. 



The positions of unstable equilibrium he found to divide themselves 

 into four special forms, the annular, cylindrical, that of a cylinder with 

 a bilateral character, and the double or multiple cylinder. The ultimate 

 form of the first case is a sphere. 



He also alluded to the interest of this fact to those who were not 

 themselves mathematicians. For the primitive forms which Professor 

 Agassiz had found to be the four types of the animal kingdom were the 

 same, the Radiata being represented by the sphere, the Mollusca by the 

 cylinder, the Articulata by the bilateral, and the Vertebrata by the 

 double cylinder. Now, as all animal forms begin as elastic sacs, con- 

 taining fluids, these forms seem the necessary ones for the condition of 

 equilibrium." 



It was Mr. Hyatt who defined the animal forms in the terms used on 

 page 164. E. S. M. 



