HORNELL— ANATOMY OF PLACUNA 67 



(c) A series of six which open into the well-defined, deeply indented biliary 

 chamber in the hinder wall of the stomach. These ducts come from the 

 posterior and ventral regions of the digestive gland. 



The ciliated epithelium of the walls of the stomach is continued a considerable 

 distance along the larger bile ducts (see Plate V., fig. 28). The gland proper is 

 composed of dense masses of secreting alveoli, lined with large cells diflPering 

 considerably in size and shape. Details are practically identical with those of such 

 other Lamellibranchs as Ostrea, Margaritifera and Cardium, and therefore do not call 

 for further detailed notice. 



The Crystalline Style has already been noted as projecting somewhat into the 

 pyloric region of the stomach. Its sac, the pyloric caecum, is wholly free from any 

 connection or fusion with the intestine along its entire course, thereb}^ agreeing with 

 those forms typified by Mytilus, Donax, and Anomia, as well as with such gastropods 

 as Pterocera, I'rochus and the FissurellidcB. 



From their respective proximal ends the pyloric caecum and intestine immediately 

 diverge, the former pursuing an anterior direction, while tbe latter in the first part of 

 its course curves posteriorly. 



The pyloric csecum (C.st.c.) bends to the right and penetrates the right mantle 

 almost immediately after leaving the stomach, and close to the anterior apices of the 

 gills. It then passes forwards and downwards just in front of the adductor muscle, 

 turns backwards at its antero-ventral curvature and then passes posteriorly parallel with 

 and to the dorsal side of the pigmented line of the median pallial sinus (fig. 13). The 

 blind termination coincides closely with the level of the normal position of the posterior 

 apices of the gills — indeed, the curvature of the pyloric caecum runs parallel with the 

 gills throughout their entire length. From its mid-length to its termination the sac 

 narrows very gradually. In preserved specimens, which naturally suff'er from muscular 

 contraction, the sac assumes a wavy or slightly sinuous course ; in the living state the 

 sac lies in a single unbroken deeply bow-shaped curve. Throughout its course the 

 csecum is embedded in an investing covering of reproductive tissue, of which a peculiar 

 spongy tissue constitutes the major portion. In section the sac is perfectly circular 

 throughout its length ; its lumen is entirely filled by the colourless glassy substance of 

 the crystalline style. In serial section of fixed material the style exhibits great 

 shrinkage and occupies but a small portion of the caecal cavity. No structure is 

 discernible save a faint concentric lamination, giving the impression that the style is 

 formed by the deposition of successive concentric layers of secretion. 



The inner surface of the stylar sac is ciliated like all other parts of the alimentary 

 canal, but the cilia are quite different from the ciliation of the other regions. They 

 are characterised by being markedly strong and densely set ; they are extremely stiff" 

 in appearance and are all of even length (fig. 25). 



G 



