ORGANS OF THE LARVA 47 



time with the velum — since it is used only when the larva is lying or 

 creeping on the bottom. In development, phylogenetically as well as 

 ontogenetically, the foot is a later organ than the velum. The velum 

 is capable of more free and rapid locomotion when the animal is small and 

 light; the foot becomes of greatest service towards the end of the free- 

 swimming period, when the animal is larger and heavier and, due to failing 

 powers of the velum, it is obliged to sink more frequently and for longer 

 periods to the bottom. Here it must often settle into soft ooze, mud, or 

 sand, or be overwhelmed with sediment, in which circumstances the foot 

 might be of life-saving value in extricating it and creeping on to a solid 

 substratum. But, judging from the relative size of the byssus-gland and 

 byssus-papilla, it would seem that the existence of the foot has a broader 

 significance, in that it may be most useful in selecting a suitable place for 

 fixation and in furnishing a byssus-secretion for the first cementing of the 

 shell fast to some rock or other object. The length and shape of the 

 byssus-papilla (heel) is sufficient, with the stretching powers of the basal 

 part of the foot, to bend round the edge of the shell and bring secretion 

 to the point of contact. In doing this the foot and byssus-gland would 

 still be preserving their original function of clinging and fixation. It is 

 conceivable that of the original, scattered, unicellular glands, that pro- 

 tected the surface against chemical action of water, some, situated along 

 the sole of the foot, became specialized to smoothen the way in creeping, 

 or were of advantage in the exclusion of water in clinging, and that,, 

 further, these became united along a common duct and sunk farther below 

 the surface to form the byssus-gland. 



That the foot Was primarily a clinging organ is supported by such 

 existing primitive mollusks as Chiton and Acmaea. Yoldia, Nucula and 

 other bivalves still retain the fiat-soled clinging and creeping foot. The 

 mussel, the clam and the oyster exhibit three different methods of special- 

 ization from such a creeping condition. The mussel becomes attached by 

 its byssus, the clam burrows, and the oyster is fixed by its shell, and as a 

 result each becomes correspondingly modified. But the young of all of 

 them still pass through a free-swimming and then a creeping condition, in 

 which there is a clinging and creeping foot provided with a byssus-gland. 

 The mussel retains its byssus organ and the reduced foot containing it, the 

 clam loses the byssus but develops the foot to a burrowing organ, the 

 oyster loses both, having become fixed by its shell. Discovery of a foot 

 for the oyster brings this genus into line with what is known about other 

 members of the group. 



Lacaze-Duthiers (1854) wrote: "En avant de I'anus un appendice peu saillant 

 simule iin rudiment de pied." From the translation by Horst (1884): "Soon the shell 

 has grown so large that it inclosed the entire body; in front of the anus there is found 



an appendage resembling a rudimentary foot The mouth seems to be placed 



between the trochal disk and the foot-shaped appendage in front of the anus; it is a long 

 funnel lined with vibratile cilia, the upper lip being formed by the disc itself, and the 

 lower lip by the appendage in question." 



