320 



COMPARATIVE ANATOMY. 



tlie two streams of water to enter and escape from it with greater 

 regularity. 



The least amount of concrescence which is observed^ gives rise to an 

 anterior larger, and a posterior smaller^ orifice (Mytilidfe) . The former 

 serves as an outlet for the foot, and as the orifice of entrance for the 

 food, while the latter, in correspondence with its position, is the orifice 

 of exit for the f cecal matters, and of the water that has served for respi- 

 ration. In the Chamacefe there are also two large openings behind 

 the anterior and larger cleft, through which the foot is protruded, and 

 which serve respectively for the entrance and exit of water ; this is 

 an arrangement which attains a higher grade of development in a 

 large division of the Lamellibranchiata (Siphoniata). That part of 

 the mantle which surrounds these orifices forms an elongated tube 

 (siphon), which undergoes other modifications in addition to its con- 

 crescence. The respiratory tubes may sometimes be formed by 



rig. 168. Lateral view of the mantle-cavity of a Ma ctr a; the right mantle-lamella 



has been removed. hr hr' Brancliias. t Tentacle, ta ir Siphons, ma Anterior, 



mp Posterior adductoi', p Foot, c Umbo. 



separate portions of the mantle ; or there may be a respiratory tube, 

 which is single externally, and is only divided internally into two 

 canals by a partition; or the two conditions may be combined 

 (Fio-. 1G8, i)' ta) ; or, finally, two completely separate tubes may be 

 developed : an upper one, the inner orifice of which is opposite the 



anus, and serves for the exit of the water, 

 and a lower one, by which the water passes 

 in. The investment of cilia causes the 

 two streams to pass regularly in and out. 

 Through these forms we are led up 

 to those in which the respiratory cavity 

 is most completely closed, and the pallial 

 tubes most developed. This is accom- 

 panied by a diminution in the size of the 

 cleft in the mantle through which the 

 foot is protruded. This has become much 

 narrower, and is placed some way from 

 the respiratory tubes, so that the greater 

 part of the edges of the mantle have grown together, in consequence 

 of which the body of the animal is sac-like (Boring Mollusca). The 

 orifice of passage for the foot is now i3laced at the anterior end, and the 



Fig. 169. The same animal 

 with its foot and siphons re- 

 tracted. ms Siphonal muscle. 



