138 Invertebrate Zoology. 



branchial sac as well as the water which flows from the two 

 large laterally lying peribranchial chambers. 



Slit the mantle from the opening already made, along 

 the upper edge of the left peribranchial cavity and thence 

 ventrally along a line midway between the endostyle and 

 the lower loop of the digestive tract, cutting through the 

 strands or trabecultz that connect the mantle with the 

 pharyngeal sac. The external wall may be now lifted and 

 turned down and the extent of the left branchial chamber 

 definitely established. On its external wall the exact course 

 of the alimentary tract will be noted from the origin of 

 the oesophagus at the point where the branchial bands con- 

 verge, to the anus. Again note the transverse vessels and 

 stigmata. 



In the same way as above directed open the right peri- 

 branchial chamber and note the course taken by the sexual 

 gland and renal organ. The two peribranchial chambers 

 are not confluent along the ventral line. 



Passing the point of a fine pair of scissors into the oral 

 siphon, carefully cut along near the ventral line of the 

 branchial sac midway between the endostyle and the ventral 

 branchial band. ^^ pharyngeal cavity will thus be opened. 

 The twelve branchial folds converge from their origin, just 

 below the peripharyngeal bands, to the opening of the oesoph- 

 agus. Each lateral half of the peripharyngeal bands, tak- 

 ing its origin from the anterior end of the endostyle, passes 

 dorsally to meet its fellow in the mid-dorsal line just back of 

 the ganglion. Thus united, the bands continue posteriorly 

 to the opening of the oesophagus as the dorsal lamina. 



The endostyle now appears as a gutter-like structure, 

 extending the entire length of the ventral side of the pharyn- 

 geal chamber. A circle of branched tentacles form a crown 



