28 



As the stomach contracts to form this division of the 

 intestine, two folds of its wall (fig. 11, PI. III.), which 

 are anterior and posterior, are formed, and are continued 

 down the straight intestine dividing the lumen of the 

 latter into two longitudinal cavities, both of which com- 

 municate with the stomach by wide openings and with 

 each other by a long wide slit. Of the two communicating 

 semi-tubes so formed, the left {Al.c.3') is the larger, and 

 is exactly circular in transverse section ; it lodges the 

 crystalline style. The right semi-tube {Al.c.3") is irre- 

 gular in section, and forms the channel along which the 

 ingested food travels. Morphologically, this is the portion 

 of the intestine immediately following the stomach. The 

 left semi-tube is a diverticulum of the stomach cavity — 

 the pyloric caecum* (sac of the crystalline style). Pyloric 

 caecum and intestine are separate in some Lamellibranchs 

 (Pholas, Donax), but in Cardium and others have fused 

 together, the anterior and posterior folds being the remains 

 of the primitively adjacent walls. At the tip of the straight 

 intestine, in the short caecum already referred to, is a 

 vestige of the originally separate condition of the two 

 channels. 



Three very distinct kinds of epithelium are present in 

 the straight portion of the intestine. On the wall of the sac 

 of the crystalline style there is a single layer of spindle- 

 shaped cells (Ep.I.l, fig. 12, PI. III.), having an average 

 height of about 0"03 mm. These bear a very close set 

 series of long and stiff cilia, having an average length of 

 f that of the cells carrying them. The cell bodies are 

 finely granular, with rather highly refractive free borders, 

 the nuclei are situated at their lower extremities ; the cells 

 fit together very closely, except at their lower extremities, 



* Purdie, A. Studies in Biology for New Zealand Students. No. 3. 

 Anatomy of the" common Mussels. Wellington, 1887. 



