128 TECTIBRANCHIATA.—PLEUROBRANCHIDA. °° 
an examination of their structure reveals particulars 
in which we see an evident departure from that 
model. The mantle is large, projecting on all 
sides over the foot, which is also ample; thus a 
deep furrow runs all round between these parts. 
Within this furrow, on the right side, les the 
branchial plume, resembling a series of pyramids, 
divided into triangular leaflets. The plume is 
attached to the body at its base, but its tip, which 
points backwards, is free. ‘The organs of repro- 
duction and of excretion are also situated on the 
same side, the former before, the latter behind the 
gill-plume. ‘The head is distinct, concealed by a 
broad veil, which is furnished with four tubular 
tentacles. The mouth consists of a simple mus- 
cular proboscis, or fleshy tube, which is capable of 
considerable elongation and contraction; it is en- 
tirely destitute of teeth or any cutting instru- 
ment, but is, nevertheless, fully able to seize and 
force into the stomach such materials as are used 
for food. The stomach is greatly developed, and 
in the principal genus is divided into four cavities, 
of which the second is muscular, and armed with 
shelly teeth; the internal surface of the third is 
increased by longitudinal plates. 
In the typical genus, there is a small thin shell 
enclosed within the mantle, and situated in the 
middle of the back, so as to cover and protect the vis- 
cera. In the curious genus Umbrella, from the Indian 
ocean and the Mediterranean, the shell is com- 
paratively large, external, and shaped somewhat 
like that of a Limpet ; but in Plewrobranchea, there 
is no trace of a shell either internally or ex- 
ternally. 
