tums sharply about 90 degi'ees across the mantle edge. 

 This point marks the upper limit oi the area 

 tliroughout which the material settled on the 

 mantle is discarded ; in the oysters which received 

 powdered suspensions large lumps of particles 

 entangled in mucus are usually seen here. A 

 similar accumulation of matei'ial ready to be dis- 

 carded marks the anterior boundary of the dis- 

 charge area which may vary in dimension and 

 location in different oysters. 



Along the ventral portion of the oyster body 

 the ciliary currents sweep across the numtle at 

 right angles to its edge and material is discarded 

 along the border. In this respect my observations 

 differ from those of Nelson (1938, p. 24), who 

 thinks that the current in this area is directed 

 along the free margin toward the mouth. Such 

 a current was not present in the large oysters 

 (C. virginica) I used in my experiments. 



The existence of a special discharge area located 

 in the zone below the labial palps is biologically 

 significant. The so-called pseudofeces, or masses 

 of discarded food particles and of detritus 

 entangled in nmcus, which accumulate in this 

 area either slide over the free edge of tlie mantle 

 and drop off or are forcibly ejected by the snapping 

 of the valves. There is no doubt that the presence 

 of pseudofeces near the edge of the mantle stimu- 

 lates the strong ejection reaction which constitutes 

 one typical pattern of shell movement in tlie 

 oyster (see p. 169). 



FORMATION AND CALCIFICATION OF 

 SHELL 



Tlie principal function of tlie mantle is the 

 formation of the shell and its calcification. Tlie 

 great structural complexity and intricate pattern 

 of pigmentation found in some species are produced 

 by the mantle. The regulatory mechanisms re- 

 sponsible for this process are not known because 

 the morphogenesis of molluscan shells has never 

 been studied experimentally. From observations 

 on shell growth in some gastropods and lamelli- 

 branchs it is clear that the shape of the sliell as 

 well as the pattern of pigmentation result from 

 the position assumed by the edge of the mantle 

 during periods of shell secretion and from the 

 rate of deposition of calcium salts and pigments. 



It can be easily observed in oysters, scallops, 

 and other bivalves in which tlie edges of the mantle 

 are not fused together that during periods of 

 growth the mantle extends a considerable distance 



beyond the border of the shell. In some species it 

 even stretches far out and folds back over the outer 

 surface of the valve. In this way, for instance, 

 the mangrove oysters produce hooks or similar 

 structures by which they attach themselves to 

 branches of trees (fig. 5). 



The differential rate of growth along the periph- 

 ery of the shell as well as the formation of spines, 

 nodes, ridges, and similar sculptural elements are 

 both caused by changes in the rate of deposition 

 of shell material. Two distinct phases may be 

 distinguished in the shell-forming process: (1) the 

 movements of the mantle which stretches and 

 folds itself in order to provide a matrix or mold 

 upon which the shell is formed, and (2) the 

 secretion and deposition of the shell material 

 itself. It is probable that the circumpallial nerve 

 plays a role in the first phase of the process by 

 controlling the muscular activity of the mantle. 

 Our present knowledge of the physiology and 

 biochemistry of shell secretion is inadequate to 

 propose an explanation of the morphogenetic 

 processes involved in shell formation. These 

 processes are not haphazard but follow a definite 

 and predetermined course. This is self-evident 

 from the fact that the final shape of the shell has 

 definite mathematical characteristics (see p. 24) 

 which can be attained only by orderly and 

 regulated deposition of organic framework and 

 mineral salts. 



The first step in the formation of the oyster 

 shell is the secretion of conchiolin from the perios- 

 tracal gland. This process can be easily observed 

 by cutting off a small section of the edge of the 

 upper valve and exposing the intact valve and the 

 underlying mantle of the opposite side. Under a 

 low-power binocular microscope one can see a 

 clear, viscous, and sometimes stringy substance 

 oozing out of the periostracal groove. While 

 secretion is taking place the edge of the mantle 

 appears to be very active, expanding and retract- 

 ing as successive layers of conchiolin are laid 

 down. Figure 91 shows the position of the mantle 

 at the time of its retraction. 



The newly deposited shell (n.sh.) extends out- 

 ward along the plane of the valve; the edge of the 

 mantle (mn.e.) rolls upvvard; its outer lobe 

 (o.mn.l.) is parallel to the plane of the valve, 

 while the middle and inner lobe (m.l.) face the 

 observer. The tentacles of the inner lobe extend 

 down; those of the middle lobe are slightly con- 

 tracted. The outer lobe underlies the sheet of 



THE MANTLE 



91 



