VII MOLLUSCA—THE SHELL 65 



The adult condition of Jmuinnctui is explained by its developmental history. 

 The shell of the young animal is like the segment of a sphere, whose greatest height 

 is hardly half of the radius. It covers the dorsal upper portion of the body, its free 

 edges thus bounding a very wide aperture, which corresponds with the anterior pedal 

 gape of PJiolas. 



In this Pholas-stsige, in fact, Jouannetia really possesses a foot. Twisting the 

 body about and rasping the stone with the anterior edge of the shell, the animal 

 excavates a hole, which is spherical in consequence of the shape of its shell. When 

 this hole is made, new accessory shell material is secreted at the free edge of the 

 shell ; this forms the "callum," and as the edge of the mantle follows the lines of 

 excavation, the form of the accessory shell is here (as in Tcirdo) determined by the 

 form of the hole, and the sphere of which the original shell was but a segment is 

 completed. 



Setting aside a few related forms {Martesia, Tercdina, Xijlopliaffa, Gastrochaena, 

 and Fistulana), in which the conditions are somewhat similar, we come to the ship- 

 worm Turcdo (Fig. 29, p. 20). This animal has a long tubular mantle which is 

 produced posteriorly in two long siphons. The body lies at the anterior end of the 

 mantle. Teredo bores cylindrical passages in wood. The valves of the shell are 

 very small in comparison with the body ; they take the form of tri-lobate pieces, 

 which encircle the anterior end of the mantle. This rudimentary shell gapes 

 anteriorly for the passage of the pestle-shaped foot, and very widely posteriorly. 

 The mantle further secretes over its whole surface a calcareous tube which lines its 

 burrow, but which does not fuse with the shell valves. Two small accessory shell- 

 pieces, the so-called " palettes," lie at the place where the siphons separate. If the 

 anterior portion of the animal reaches {i.e. if it bores through to) the water, the 

 calcareous tube is rounded off and closed. 



Aspergillum {Brechiics, Fig. 30, p. 20, and Fig. 65) and Clavagella show similar con- 

 ditions. In the club-shaped shell, which inserts its anterior thicker end into rock, 

 shell, coral, or sand, we can distinguish a true and a false shell. The false shell 

 forms by far the larger portion of the tube, and corresponds with the secreted tube 

 of Teredo, and with a callum like that of Fholas. The true shell is very small 

 and lies anteriorly. The two valves of this true but rudimentary shell are, in 

 Aspergillum, placed saddle-like over the anterior end of the tube, with which they 

 are firmly fused (Fig. 30, p. 20). Were they isolated, their gape would be 

 unusually wide, not only anteriorly and posteriorly, but ventrally. The shell-tube is 

 open posteriorly, over the apertures of the siphons ; anteriorly, however, it is closed 

 (in the adult) by means of a disc perforated like the rose of a watering-can, which 

 corresponds in position with the callum of the Pholadidcc. The perforations at the 

 edge of the disc, or even over its whole surface, are sometimes produced into cal- 

 careous, and at times dichotomously branched tubules. In the middle of the disc 

 there is sometimes found a narrow slit-like aperture corresponding with the pedal 

 aperture in the mantle beneath, but this is often wanting. Less frequently, we find 

 another aperture in the ventral middle line, corresponding with the fourth mantle 

 aperture above desci'ibed (p. 51). 



Aspergillum buries its anterior end in mud or sand, but its whole organisation, 

 and especially its shell arrangement, point to a former boring mode of life. 



Clavagella, which is nearly related to Aspergillum, bores into rock or the cal- 

 careous shells of various other animals. The arrangement of its shell differs from 

 that of Aspergillum chiefly in the somewhat greater size of its true valves, and in 

 the fusion of only the left valve with the calcareous tube, the right lying free 

 within that tube. 



In the Pholadidcc, the ligament, which is still found at the hinge, no longer acts 

 for opening the shell. In consequence of a peculiar arrangement of the anterior 

 VOL. II F 



