48 COMMISSION OF CONSERVATION 



If we refer to Horst's (1884) Fig. 15 or 16, or to Huxley's (1883) Fig. 3 A, it will 

 appear altogether probable that the foot-like appendage was in reality the lower lip, 

 as in fact Lacaze-Duthiers states. There is nothing significant about the expression, 

 "in front of the anus," since at this period mouth and anus are not far apart, and the 

 thickened lower lip is prominent. All the references to velum (trochal disk), shell, 

 mouth, stomach, liver-granules, intestine, muscles, otoliths, or other organs, refer to 

 a larva in the young straight-hinge stage, when a foot is not yet developed. It must be 

 remembered that the only method of procuring larvae at the time was by taking them 

 from the parent European oyster, and that rearing from eggs and capturing in plankton 

 were unknown, as were also the older straight-hinge and all the umbo stages, and it 

 was not suspected that larval life continues for so long a time as it does, or can be di- 

 vided into clearly marked periods. A young larva is clearly referred to in the stiff, 

 fixed type of trochal disk: "Even in the more developed larvae neither branchia nor 

 heart could be observed, not any movement of the trochal disk as stated by Davaine." 



Brooks (1880) made the statement: "Near the centre of the ventral surface — the 

 top of Figure 32 — there is a well-marked and constant protuberance of the body wall, 

 which occupies the region which, in most molluscan embryos, gives rise to the foot, and 

 which may perhaps be regarded as a rudiment of that organ." In the same paragraph, 

 and referring to the same figure, he mentions "the primitive digestive cavity", and on 

 page 6S "the primitive digestive tract opens by a wide blastopore," while on page 54 

 referring to a slightly later phase he said: "The foot-like protuberance on the ventral 

 surface has disappeared, and the blastopore on the dorsal surface has entirely closed." 

 No one would claim that the part referred to in these extracts is the same organ as I 

 have described in very much later larvae. The protuberance could have nothing to do 

 with the foot. It disappeared long before the true foot was developed. It was on the 

 dorsal surface (not ventral as Brooks thought at the time). Even if we admit that it 

 was ventral a comparison with Fig. 37 will show that it was situated in front of the 

 mouth, which could not be possible with the foot. That Brooks changed his view may 

 be seen in his book of 1905, Fig. VIII, where the depression behind it is regarded as the 

 shell-gland. Moreover it would precede the shell in time and be the first molluscan 

 character to appear. 



Horst (1884) stated: "The portion on the ventral side, situated below the mouth, 

 now begins to protrude very strongly, so as to form a sort of foot which causes the 

 embryo to resemble a young gastropod. The blastopore continues to be very distinct." 

 Referring to his Figs. 9, 10, we observe that it is only an accidental prominence, since 

 it is bounded below by the invagination of the blastopore and above by that of the shell- 

 gland, and further, it disappears later on as in his Figs. 13, 14. The so-called foot of 

 Lacaze-Duthiers, of Brooks, and of Horst are three different parts. 



Jackson (1890): "The nearest approach to a foot known in the developing oyster 

 is that shown in Fig. 24, p. 299, and I discovered no traces of a foot in my youngest 



specimens The fact that a velum, or swimming organ, exists up to the 



period of permanent fixation, accounts for the great reduction of the foot, because that 

 organ is unnecessary while the animal is provided with another locomotive organ, and 

 is useless for progression after the animal is permanently attached." The figure re- 

 ferred to is that of Horst, Fig. 15, where the reference is clearly to the lower lip, at a 

 stage considerably later than that for which Horst claimed a "pediform appendix." 



The best that can be said for all references to a foot in these early stages is I hat, by 

 comparison with other species, they indicate the place where, at a later date, through 

 growth and specialization, a foot as well as several other parts are formed between the 

 mouth and the anus; zoologists by inference from comparative embryology were pre- 

 pared to find a rudiment or a vestige of this very characteristic molluscan organ. 



Mantle. — The mantle continues to grow downwards as two fleshy 

 folds right and left of the body (Plate VI, figs. 1-9), to which they are 

 attached along the dorsal region. The lateral flaps are thin and the 

 margins free below, in front and behind. They lie against the inner sides 

 of the shell-valves and are responsible for the growth of the latter. The 

 edges form a thickened rim, containing unicellular glands, and supporting 

 irregular processes resembling tentacles, that sometimes protrude beyond 

 the margin of the shell. 



