LECTURES ON EMBRYOLOGY. 



, which comes out from the mouth. We 

 enight not expect to find these eyes abottt the head 

 in any way analogous to the large number ef eyes 

 ^vhieh surround the margin 07" Che mantle. 



Nevertheless, if I have understood he structure 

 of MollKsca,! shall show that these eyes are .11 the 

 same as those of the Oyster, the same as those of 

 the Cuttle Fish, the same as those of the Strotnbus, 

 the same as those of all ether Mollusea. And I 

 will try to reduce all these different forms to a few 

 simple types, and then compare these few simple 

 types together, in order to find, if possible, &e 

 common uniform type. The Scallop, which have 

 already mentioned, belongs to the so called bivalv- 

 -ad shells to the Acepiiala. And there are many 

 kinds with regular or irregular shells, the two 

 valves being equal in some, as is the Clam (Plate 

 XXXV, fig. B) for instance, and ia other hard- 

 shelled animals; one being deeper than the other, 

 one exceeding the other, and forming a beak over 



it, or being unequal, as the Oysters are unequal, 





 [PLATE XXXV. ACE?HALA CLAMS.] 



All th^se differences will not modify the general 

 arrangement of parts as seen in tbe Scallop. 

 There is a mantle around the whole body; next two 

 pairs of gills : the fleshy mass is in the centre ; 

 and above are grouped all organs, as the liver, ali- 

 mentary canal, &C M &c. (Plate XLIV, fig. A ) 

 11 



[PLATE XXXIV GASTEROPODA.] 



In the Snail-like Mollusea, or Gasteropoda, we 

 have, on the contrary, a large fleshy mass below, 

 on which the animal walks. At the anterior part 

 of the body, there is a pair of eyes upon tentacles 

 a-nd above the foot, the main mass of organs the 

 stomach, t-iie liver, the gills generally protected 

 by the shell. If the shell be removed and the man- 

 tle split, we have the gills on one side, the liver on 

 the other, the intestines winding within, the heart 

 being near by, and the whole mass within the shelU 



But anaong these Gasteropoda or Snail-like ani- 

 mals, there are many in which the body is not so 

 complicated, or at least net twisted, but straight as 

 in Eolis, Boris, Patella, Chiton, Emarginula, Fis- 

 surella, &c. &c. And in some of them we see that 

 the body, for instance in Eolis, (Plate XLII fig. C) 

 has respiratory appendages symmetrically on both 



[PLATE XLV. TERBRATULA AND SPIRIFERA.] 



