176 IOWA ACADEMY OF SCIENCES. 



side of a lamella may be seen, may aid in understanding the 

 structure of a gill. The descending and ascending portions of 

 each filament, / i I, are fused throughout their length, thus 

 uniting the lamella? at very short intervals and restricting indi- 

 vidual water-tubes, 20 t, between adjacent filaments. 



The filaments are strengthened by chitinous rods, c r, and 

 attached to one another laterally by inter-filamenter junctions, 

 ■^ / J, which are places where, during development, adjacent 

 filaments have fused together. There are thus left openings, 

 i o, known as inhalent ostea, which lead into the water-tubes. 

 Beneath the epithelial covering of the filaments is a loose con- 

 nective tissue, through which more or less definite blood spaces, 

 & I s, may be traced. The outer surfaces of the filaments are 

 covered with rather short cilia, besides which there is a row 

 of longer cilia on each side of each filament near the outer sur- 

 faces, and another row of long cilia placed far in on the sides 

 of the filaments, nearly opposite the chitinous rods. It seems 

 that the inner rows of cilia serve largely to drive the water 

 through the inhalent ostea and water- tubes and thus keep up a 

 continuous supply of fresh water, while the other cilia are 

 engaged in forming surface currents and in separating and 

 transporting food particles. 



LABIAL PALPI. 



The labial palpi, Pig. 3, I p, are very long and slightly 

 curved. There is a pair, consisting of an outer and an inner 

 palp, on each side of the body. The anterior edges of the outer 

 palps are connected in front of the mouth by a slight ridge, as 

 are likewise the anterior edges of the inner palps behind the 

 mouth. The adjacent sides of each pair are grooved and 

 densely ciliated. Particles of food passed between them from 

 the gills are transported to the mouth. 



ALIMENTARY CANAL. 



The mouth, situated behind the anterior adductor muscle 

 leads into a rather long and slender oesophagus. Pig, 1, o e, 

 which communicates with a somewhat spacious horn-shaped 

 stomach, sacculated at its upper end, which curves downward 

 and forward and gradually tapers into the intestine which at 

 this point forms a coil. The relative positions of the loops of 

 this coil to one another, may be made out by comparing Pig. 

 1, with Pig. 4, which latter represents an obliquely transverse 

 section through the coil. The stomach 1, situated on the left 



