EMBRYOLOGICAL SYSTEMS. 355 



and the nervous system, extends through the whole length 

 of the body, and all organic motion in these animals has 

 the same prevailing direction. Only subordinate branches 

 of these organs arise laterally, and chiefly wherever the 

 general contrast, manifested in the whole length, is re- 

 peated in such a manner, that, for each separate segment, 

 the same contrast arises anew in connexion with the essen- 

 tial elements of the whole organism. Hence the tendency 

 in these animals to divide into many segments, in the 

 direction of the longitudinal axis of the body. In the 

 true Insects undergoing metamorphosis, these segments 

 unite again into three principal regions, in the first of 

 which the life of the nerves prevails; in the second, mo- 

 tion ; in the third, digestion ; though neither of the three 

 regions is wholly deprived of any one of these functions. 

 Besides the opposition between before and behind, a less 

 marked contrast is observed in a higher stage of develop- 

 ment between above and below. A difference between 

 right and left forms a rare exception, and is generally 

 wanting. Sensibility and irritability are particularly de- 

 veloped in this series. Motion is active, and directed 

 more decidedly forward in proportion as the longitudinal 

 axis prevails. When the body is contracted, as in spiders 

 and crabs, its direction is less decided. The plastic organs 

 are little developed; glands, especially, are rare, and 

 mostly replaced by simple tubes. 



III. The Massive Type. We may thus call the type 

 of Mollusks, for neither length nor surface prevails in 

 them; but the whole body and its separate parts are 

 formed rather in round masses, which may be either hol- 

 low or solid. As the chief contrast of their structure is 

 not between the opposite ends of the body, nor between 

 the centre and periphery, there is almost throughout this 



AA 2 



