BIVALVE MOLLUSCANS. 235 



we have just left, as we have seen, appears of a 

 higher character than that of any of the preceding 

 ones ; traces of a heart appear ; a nervous ganglion 

 is detected between the mouth and anus, sending 

 nerves to each ; a regular respiratory system by 

 means of gills becomes evident ; but still the 

 animal is furnished with no head, no eyes, and in 

 numerous cases has no separate existence, but 

 forms a branch of the general body thus resem- 

 bling a plant from which it cannot dissociate 

 itself and become an independent individual. 



Indeed when we enter the Class of Molluscans, 

 we find that the nearest affinities of the Tuni- 

 caries have likewise no head, and this circum- 

 stance appears to have induced Lamarck not 

 only to separate them from the class as arranged 

 by Cuvier, but also his whole family of headless 

 Molluscans, 1 of which he forms his two Classes of 

 Cirripedes* and Conchifers. 3 The absence of a 

 head from the animals of the bivalve and 

 multivalve shells, is certainly a circumstance 

 which, at the first blush, appears to justify 

 their separation classically from the other Mol- 

 luscans, but when we compare other characters, 

 we shall find many that are common to both, 

 particularly their nervous system, which is 

 the same both in the Conchifers and Mollus- 

 cans of Lamarck ; for neither of these exhibit 



1 Mollusca acephala. 2 Cirripeda. 



3 Cone hif era. 



