CONCHIFERA. 389 



membranous tubes of considerable length called syphons, through 

 which the water is conveyed to the gills, and excrementitious mat- 

 ters expelled from the body. In the Mussels (Mytilacea) the 

 edges of the mantle are partially joined so as to present two aper- 

 tures, through one of which the foot is protruded, while the other, 

 the smaller of the two, gives issue to the excrement. A third family 

 (Camacea) has the circumference of the two divisions of the mantle 

 still more intimately united, leaving three distinct fissures, one 

 for the passage of the foot, another for the entrance of water to the 



Fig. 179. 



branchise, and a third for the ejection of matter from the rectum. 

 Of these, some are of gigantic dimensions, and fix themselves by 

 a strong byssus. One species, indeed, (Tridacne gigas,) is so 

 enormous in its size, that its shells alone not unfrequently weigh 

 upwards of two hundred pounds, and hatchets are employed to 

 chop its thick and tendinous cables from the rock to which it holds. 

 The Cockle family (Cardiacea) is recognised by having the 

 mantle open anteriorly, but prolonged at one extremity into two 

 tubes, one of which admits the water for respiration, while the 

 other discharges effete matter. In the Cockle (Cardium) the tubes 

 are short, and scarcely reach beyond the shell (Jig. 181, a) ; but in 

 other genera, as, for example, Mactra (Jig' 179, &, c), they are of 

 such length, that, when extended, they protrude to a considerable 

 distance. We at once perceive the use of the tubular arrangement 

 of the mantle here referred to, when we reflect upon the already 

 mentioned habits of this extensive division of the Conchifera, and 

 consider how, by means of their largely developed foot, they burrow 

 into the sand or mud of the shore. Had their mantle been open, 



