176 
IOWA ACADEMY OP 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, fil^ are fused throughout their length, thus 
uniting the lamellse at very short intervals and restricting indi- 
vidual water-tubes, lo t, between adjacent filaments. 
The filaments are strengthened by chitinous rods, c r, and 
attached to one another laterally by inter-filamenter junctions, 
i f j, which are places where, during development, adjacent 
filaments have fused together. There are thus left openings, 
i 0 , 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. Fig. 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. Fig. 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 Fig. 
1, with Fig. 4, which latter represents an obliquely transverse 
section through the coil. The stomach 1, situated on the left 
