Chap. III. EMBRYOLOGICAL SYSTEMS. 223 



always at opposite ends, and usually also the sexual organs, tliough their opening 

 is sometimes farther forward ; this occurs, however, more frequently in the females, 

 in which these organs have a double function, than in the males. When both 

 sexual organs are removed from the posterior extremity, the opening in the female 

 usually lies forther forward than in the male. So is it in the Myriapods and 

 the Crabs. The Leeches and Earthworms present a rare exception. The recep- 

 tive pole being thus definitely fixed, the organs of senses, as instrumental to the 

 receptivity of the nervous system, early reach an important degree of perfection. 

 The intestinal canal, as well as the vascular stems and the nervous system, extend 

 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 repeated in such a manner that, for each separate segment, the same 

 contrast arises anew, in connection with the essential elements of the whole organ- 

 ism. 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, motion ; 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 development between above and 

 below. A difference between right and left forms a rare exception, and is gen- 

 erally wanting. Sensibility and irritability are particularly developed in this series. 

 Motion is active, and directed more decidedly forward, in proportion as the lon- 

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

 its direction is less decided. The plastic organs are little developed ; glands, espe- 

 cially, are rare, and mostly replaced by simple tubes. 



III. The Massive Type. We may thus call the type of MoUusks, for neither 

 length nor surface prevails in them, but the whole body and its separate parts are 

 fonned rather in round masses which may be either hollow 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 type an absence of sym- 

 metry. Generally the discharging pole is to the right of the receptive one. 

 The discharging pole, however, is either near the receptive one, or removed from 

 it, and approximated to the posterior extremity of the body. As the tract of 

 the digestive apparatus is always determined b}' these two poles, it is more or 

 less arched ; in its simplest form it is only a single arch, as in Plumatella. 

 When that canal is long, it is curled up in a spiral in the centre, and the spiral 

 probal)ly has its definite laws. For instance, the anterior part of the alimentary 

 canal appears to be always placed under the posterior. The j^rincipal currents 



