64 



COMPARATIVE ANATOMY 



CHAP. 



Ephippium. 1. Hinge edj 

 pression of adductor. 



e : 2, irii- 



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, aud more or less removed from it, we tiud 

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

 line, caused liy the nmscle 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 

 liends forward and upward, and then again passes 

 backward and upward towards the loAver 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 {Siniqxilliatii, latcgripaJ- 

 liafii, 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 Pema these retractors and the 1 letter developed the siphons 



the larger and clearer is the sinus. 



3. The foregoing imjiressions 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 Lamellibrancliia, when the shell is closed, the edges of the two valves 

 meet exactly, so that the soft bofh" 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 

 (r.;/., Mijidcr, (Hyc!imri-iih:i\ Soh'iiiOw). This is accoimted 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 or with difficulty be withdrawn into the shell.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, Fholadidfn, 

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



The shell of Pluilns is elongated longitudinally, and gapes anteriorl}- and ventral! v 

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

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

 (prosoplax, mesoplax, metaplax). 



The shell of Pholadiden somewhat resembles that of Pholas. In the yount; 

 animal it gapes anteriorly, as in Phulns, for the passage of the foot. Posteriori}', 

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

 piece (siphonoplax), hollowed out like a trough. The siphonojilax 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 meso- and metaplax are 

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

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

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

 in the substance into which it has bored. 



The shell of the adult Joiumnrtvi. 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 lie 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 (c/. also Figs. 27, 28, p. 19, and 66, p. 67). 



