August 14, 1896.] 



SCIENCE. 



189 



rangement is that the foot moves about a 

 point near the common axis of the animal 

 and its burrow and not at one side, thus 

 enabling the foot to work with equal advan- 

 tage in all directions. 



The mantle lobes are fused, except in the 

 antero- ventral region (and, of course, at the 

 siphonal openings), where an opening is 

 found sufficiently large for the protrusion of 

 the foot. The edge of the mantle opening 

 seems to be provided with circular muscle 

 fibres, by means of which the opening is 

 kept just large enough for the foot. Speci- 

 mens of this form when removed from the 

 burrow will be found to have the foot armed 

 with grit, and a glance at its size and mus- 

 culature is enough to convince one that it 

 is functional. 



These characters remain unchanged until 

 the work of drilling the burrow is com- 

 pleted. The depth and size of the hole will 

 be determined by the number and frequency 

 of the neighboring burrows or the hardness 

 of the rock. Assuming that the work is 

 finished and the author is ready to rest 

 from his labors, let us see what changes 

 take place. The foot, being thereafter of no 

 use, begins to atrophy, till at last the mus- 

 cular tissue is entirely lost, the whole bulk 

 being taken up by the genitalia and diges- 

 tive tract. As the foot disappears, the 

 opening through which up to this time it 

 has protruded grows smaller by further 

 fusion of the mantle lobes, till there is left 

 at the extreme anterior end a small opening 

 about one and a half mm. in diameter. 

 This opening is supplied with a sphincter 

 muscle and two valves on the inside, so 

 placed that egress of water at this point 

 may be prevented. The opening seems to 

 be used in drawing in water and debris 

 chancing to be in the burrow. 



As this fusion takes place the gape of the 

 shell becomes closed by plates secreted by 

 the antero-dorsal mantle folds above and by 

 the thick mantle now closing in front of the 



foot below. This results in the complete 

 armature of the anterior end of the animal. 

 At the posterior end an extra length of 

 shell and two cuticular flaps, leathery con- 

 tinuations of the shell, are produced. The 

 shape and size of these is such that the bur- 

 row is completely closed at this point by 

 them as by valves. After these changes 

 have taken place there is no more boring 

 done, and we have now the fully matured 

 but degenerate animal. It is interesting to 

 note that not infrequently other clams 

 Saxidomus are caught when very small in 

 the burrow of the Piddock. Such forms 

 have, it is well known, a very strong and 

 muscular foot, which, however, becomes 

 functionless in imprisonment and dwindles 

 away until it is almost, if not entirely, lost. 

 The shell becomes elongate or otherwise 

 changed from the normal shape by pressure 

 of the walls which imprison it. Mussels, 

 too, attach themselves by their byssus to 

 the wall of the burrow near its mouth, 

 where it is narrow, and become much elon- 

 gated. 



The absence of an elastic hinge ligament 

 is a striking character, not only of this 

 form, but of Piddocks in general, and of an- 

 other allied form. Teredo. The valves are 

 held in position each relatively to the 

 other by the common cuticular invest- 

 ment, which is, however, rather thicker and 

 stronger along the dorsal line. The point 

 at which the valves actually come in con- 

 tact is morphologically the outside surface 

 of the umbo, thus forming a double ball joint 

 about which the dorsal cuticle is so dis- 

 posed as to give rise to an incomplete cap- 

 sular ligament. In place of the hinge liga- 

 ment we find that the anterior adductor 

 muscle, instead of remaining inside the 

 shell, extends backwards and dorsal to the 

 umbones, so that contraction of this muscle 

 does not result in closing the valves, as it, 

 aided by the posterior adductor, does in other 

 dimyarians, but in separating them ven- 



