368 



MOLL USC A. 



Cardium*, Serripes*) and ASTARTID^E, marine, and the fresh-water CYCLA- 

 DID.E (Cyclas, Pisidium *), about the size of peas, belong here, as probably 

 do the extinct RUDISTID.E of the cretaceous. 



Sub Order II. SINUPALLIATA. The VENERID.E with swollen shells, 

 represented by the quahog, Venus mercenaria on our east coast and by 

 brightly colored species in the tropics; the MACTRID^E or 

 hen clams, and the flattened delicate TELLINID^E (Tellina*, 

 Macoma*), have short siphons. In others the siphons are 

 so large that they cannot be entirely retracted within the 

 shell. This is the case in the MYID^E, represented in all 

 northern seas by the long clam, Mya arenaria, and in the 

 razor clams (SOLENID^E; SolenEnsatella*). The allied SAXI- 

 CAVID^ have burrowing species. These forms connect 

 with others in which the united siphons far exceed the rest 

 of the body in length, giving the animal a worm-like ap- 

 pearance (fig. 359). Since the valves do not 

 cover the whole shell, they are supplemented 

 by accessory shells, or the worm-like body 

 secretes a tube in which the rudimentary 

 valves are imbedded (fig. 360). The PHO- 

 LADID^E, some of which are phosphorescent, 

 burrow in wood, clay, or stone. The shell is 

 well developed. In the ship worms (TERE- 

 DHXE) the shells, on the other hand, are small, 

 while in some species the burrows made by 

 these animals in wood are lined by calcareous 

 deposits. The several species of Teredo* by 

 their boring habits do much damage to wood 

 in the sea, especially in the tropics. The 

 GASTROCH^ENID^E also form tubular shells, the 

 valves being imbedded in the tube (fig. 360); 

 at the smaller anterior end the tube is open, 

 but the other end is closed by a perforated 

 plate, giving these animals the name of 

 * sprinkling-pot ' shells. 



Lastly, there should be mentioned the 

 Septibranchiata, in which the 



have the sha P e of a se P tum perforated 

 LudWig-Leu- by gill slits separating the branchial and clo- 

 chambers> si i enia ^ Cuspidaria. 



B 



FlG. 359. Te- 

 redo navalis, 

 ship worm in 

 its tube, the 

 siphons (a, 

 anal; b, bran- 

 chial) drawn 

 out of the 

 tube (r); k, 

 shell. B,teeth 

 of the shell 

 enlarged. 



Ispe5$? little-known 



nis.) a, shell. 



