LECTURES ON MOLLUSCA. 247 



not speak or write in diverging lines, it is more convenient to begin 

 with the borers, although they are in many ways abnormal. 



Several of the Lamellibranchs are now known to have the sexes sep- 

 arate, like the trunk-bearing univalves. As the individuals always 

 maintain a solitary existence, it is probable that the fecundating influ- 

 ences are diffused and inhaled through the breathing currents. The 

 eggs are matured between the outer plates of the gills. The young 

 always swim freely about, by means of a hairy flap, which disappears 

 when the foot is developed, at the front of which is a slender tail. At 

 this time they have minute eyes, which disappear as the animal hides 

 itself within its wings. It is singular that in the last published 

 treatise, these creatures are said to be self-impregnating hermaphro- 

 dites ; although the difference of shape between the shells of male and 

 female specimens has often raised them to the rank of different spe- 

 cie's. 



It is evident from the essential conditions of life in these headless 

 inollusks, that their structure could not be modified to exist on land, 

 like the Pectinibranchiate and Pulmonic Snails. A very few of the 

 plate-gilled families are able to exist in fresh waters ; but the whole 

 of the other classes are marine. 



Family PholadidjE. (Piddocks or Date-Fish.) 



If we divide the ordinary bivalves into active or sedentary, accord- 

 ing to their habits of life, we shall find among the latter the two most 

 widely divergent groups — the oysters, which sleep on their sides, and 

 the borers, which stand on their feet. The habits of the borers have 

 been already described at some length, (v. Smithsonian Eeport for 

 1859, pp. 209-217 :) it will be sufficient here to point out the principal 

 differences of structure. The Piddocks have white shells ; generally 

 very thin, but strong, and adorned with rasp-like sculpture. As this 

 sculpture however is for the most part turned towards the aperture, it 

 cannot be much used for excavating the hollows. The naturalist who 

 took the trouble to bore a hole with the shell, could do so most easily 

 if he turned the shell the wrong way in. As before stated, the stout 

 club-shaped foot is probably the principal instrument of abrasion. This 

 is fixed by strong muscles to the shell, which has no articulated hinge 

 and ligament, like other bivalves, but is strengthened by a spoon- 

 shaped process, curling up from within the beaks. The pipes are 

 long, united till near the ends, and inclosed in a tough skin which is 

 often protected by cartilaginous " cups." The shells gape all round, 

 except at a point before and behind, and the vacant spaces are gene- 

 rally covered, in the adult, by accessory plates ; which caused Linmeus 

 to separate them from their allies as being " niultivalves." They 

 are phosphorescent, living by their own light ; and are often eaten as 

 a delicacy. Plwlas proper has one shield placed behind the hinge. 

 Dactylina has a shield over each valve, a cross piece, and a long plate 

 along the back. ZirpJuea has a broader shell without plates : it is 

 the only one of the British species which is also found in America. 

 The little group Navea are slightly modified to suit their residence in 

 sponge. Xylophacja looks like a very short Ship-worm, making bur- 



