68 



h. The Alimentary Canal. 



In adaptation to their burrowing mode of life, the 

 alimentary canal of all the Pholadacea h.as liooonie nore 

 highly specialized, perhaps, than in any other type of 

 Lamellibranch. Th.is specialization is carried farthest 

 in the "Ship-worne".' . 



I'lost of the parts of tl:e alimentary canal of the 

 adult are already present in the newly-attached larva, 

 though their relations to each other and their relative 

 development are very different. The general plan is shovm 

 in fig. 2, which represents a larva from the rirrht side 

 with the shell, mantle and gills removed. A long cil- 

 iated oesophagus (figs. 2,24, oe.) leads into a rather 

 small stomach, from which project on either side the two 

 large, simple liver lobules, composed of large^ coarsely 

 granular, pigmented, non-ciliated cells (figs. 2, 25). The 

 intestine leaves the right side of the stomach, (figs. 2, 

 26) and after forming a single loop passes over the poste- 

 rior adductor a3 the rectum. Just posterior to the intes- 

 tinal opening is a small hemispherical diverticulum of the 

 stomacli, the caecum (ce. figs. 2,25,26) composed of dense- 

 ly granular, non-ciliated cells. The posterior, ventral 



