64 COMPARATIVE ANATOMY CHAP. 



is always to be found close to the posterior (which in the Monomyaria is the only) 

 adductor. 



2. Parallel to the edge of the shell, and more or less removed from it, we find 

 on the inner surface of the shell the so-called pallial 

 line, caused by the muscle fibres which attach the 

 edge of the mantle to the valves. 



The course taken by this line undergoes charac- 

 teristic modification in such Lamellibranchs as have 

 siphons ; at the posterior part of the shell it suddenly 

 bends forward and upward, and then again passes 

 backward and upward towards the lower edge of the 

 posterior adductor. The pallial line, in this case, 

 forms an indentation, leaving a sinus or bay opening 

 posteriorly, the pallial sinus, which has been utilised 

 for systematic purposes (Sinupalliata, Integripal- 

 liata, Fig. 63). The sinus marks the line of attach - 

 Fio. 64. Monomyarian, internal ment of the sipho-retractor muscles ; the stronger 

 surface of a shell valve of Perna these retractors and the better developed the siphons 

 Ephippiuin l. Hinge edge; 2, im- th ^ &nd dearer ig the ^ 

 pression of adductor. E.. . 



3. The foregoing impressions are the, -most dis- 

 tinct and constant, but others may occur as well, caused by the protractors and 

 retractors of the foot, by the muscles or ligaments which attach the visceral dome 

 to the shell, etc. ; but these cannot be further described. 



In most Lamellibranchia, when the shell is closed, the edges of the two valves 

 meet exactly, so that the soft body can be entirely enclosed and cut off from the 

 exterior (closed) shell. There are, however, shells in which, in the closed condition, 

 the valves gape posteriorly, or, more frequently, both posteriorly and anteriorly 

 (e.g., Myidce, Glycymeridce, Solenidce). This is accounted for by the great develop- 

 ment of the siphons and of the foot, which can only partially (Myidce, Solenocurtus] 

 or with difficulty be withdrawn into the shell. Such gaping shells are found in most 

 boring bivalves, whose shell formation is specially interesting owing to the develop- 

 ment of accessory valves or calcareous tubes. In this respect Pholas, Pholadidea, 

 and Jouannetia represent the most important stages in a remarkable series. 



The shell of Pholas is elongated longitudinally, and gapes anteriorly and ventrally 

 for the passage of the short club-shaped foot, and posteriorly for that of the strongly 

 developed siphons. As many as three accessory valves are developed dorsally 

 (prosoplax, mesoplax, metaplax). 



The shell of Pholadidea somewhat resembles that of Pholas. In the young 

 animal it gapes anteriorly, as in Pholas, for the passage of the foot. Posteriorly, 

 each valve is produced into a horny process, which is succeeded by an accessory 

 piece (siphonoplax), hollowed out like a trough. The siphonoplax of the one valve 

 often fuses with that of the other to form a single tube for the reception of the 

 siphons. There are two pieces of prosoplax, while the nieso- and metaplax are 

 rudimentary. In the adult the boring activity is suspended, and the anterior 

 opening becomes entirely closed by the secretion of an accessory piece, the callum 

 (hypoplax). The functionless foot atrophies, and the animal can move no farther 

 in the substance into which it has bored. 



The shell of the adult Jouannetia is much shortened longitudinally, and is 

 globular, and the animal cannot move in the round hole it has bored for itself in a 

 block of coral. Any alteration in its position in the hole, which might be fatal to 

 the animal, is avoided by means of a posterior tongue-like process of the shell, 

 which, however, only belongs to the right valve. The shell is completely closed 

 anteriorly, and a foot is wanting (cf. also Figs. 27, 28, p. 19, and 66, p. 67). 



