215 



were waslird mit witli a pipette. Small quantities of mercuric sulphic.le and (it tine 

 carborundum dust were placed on \arious parts of the wall and on the typhlosole. 

 There was no indication of any ciliary action, thonsfh such activity was very evident 

 in other parts of the stomach and in the pallial cavity under like conditions. Slow 

 writhing movements of the tj-j^hlosole were observed, however, and it is apparent 

 that material is moved in and out of the caecum by muscular movements of the walls 

 rather than by ciliary action. The experiment was repeated on several animals with 

 similar results. This muscular movement approaching peristalsis is an iniusua! develop- 

 ment in this order of lamellibranchs. 



The orifice between the caecum and the stomach (c.o.) is inconipleteb- di\i(ied 

 into two openings by two lateral inloldings oi the w^dl. The right fold is continuous 

 with the caecal typhlosole. This arrangement probably provides for simultaneous 

 ingress anfl egress of wood chips through the two di\isions thus formed. 



Deshayes (1848) and Quatrefages (1849) describe a caecum strikingh different 

 from that described here and l)y other authors. Deshayes calls it the second stomach. 

 It is thin walled, with a narrow orifice, and with a typhlosole ("valvule") like a funnel. 

 This second stomach doubles back on itself and ends blindh'. It is buried in brown 

 hepatic tissue. The intestine takes off from it. Quatrefages goes into less detail in 

 describing his species, but his figures indicate essentialK- the same structure. He 

 denies, however, that the intestine leaves the caeciuii and lielie\es this statement to 

 be an errtir on the part of Deshayes. 



The sac of the crystalline style (s.c.s.. figs. 77, 78) lies in a transverse position in the 

 lower part of the foot, opening into the stomach on the left. The whole mechanism 

 of the cr\stalline style is essentially similar to that descrilied by Nelson (1918) for 

 other lamellibranchs. The sac is pyriform and the interior is covered with the char- 

 acteristic dense mat of long cilia. Extending from the mouth of the sac to its distal 

 end is a small groove in the wall with a slight ridge on one side of it. The cells along 

 this groo\e and ridge differ from those of the rest of the sac in that they stain deeply 

 with Delafield's hematoxylin, are narrower, bear shorter cilia or none at all, and have 

 their nuclei close to the basement membrane instead of near the center or toward the 

 distal ends. From Nelson's descriptions of other species, we conclude that these 

 cells are the ones which secrete the substance of the crystalline st>le. This groove 

 terminates distally in the orifice to a small appendix to the sac. This appendix is 

 termed by Sigerfoos (1908) the tubular part of the sheath of the crystalline style. 

 Its thin walls are made up of non-ciliated cells similar to those in the secretory groove 

 and ridge, and are continuous with them, indicating that it contributes to the forma- 

 tion of the style, as Sigerfoos suggests. A large fold of the epithelium overhangs the 

 opening of the sac into the stomach. 



The crystalline style itself is of the usual clear, gelatinous consistency. It is 

 shaped like a hea\y club, the smaller or "handle" end of the club projecting across the 

 lumen of the stomach to the opposite wall where it bears against a structure which 

 Nelson (1918) terms the gastric shield. This shield is a thin, transparent, cartilage- 

 like plate secreted by the epithelium of the stomach. It may be dissected out or may 

 be seen in microscopic sections. Its posterior and dorsal edges are marked by a faint 

 ridge on the wall of the stomach. Ventrally it extends to the mouth of the sac of the 

 st\le, and anteriorly it reaches into the lateral pouch and the dorsal caecum. 



The observations we have recorded here agree in all essentials with Nelson's 

 (1918) descriptions of species in which the sac is entirely separated from the intestine. 

 He found that the style is rotated by the cilia lining its sac and that it is gradually 

 dissolved away. It thus helps to keep in motion the contents of the stomach, and the 



