INTRODUCTION. XXlil 
dage; comprising two forms moulded on the type of the 
two preceding ; the shorter changing the hooked extremity 
for a bulbous termination, and the shaft being armed with 
teeth on one side only. 
A seventh exists on the mandibular appendage: it is 
straight, enlarged and rounded at the apex, and serrated 
on one side; while 
An eighth differs from the preceding in being more 
robust, slightly turned at the extremity, and smooth along 
the margins, excepting a single short, straight, distally 
directed cilium. 
A ninth resembles the sixth, but wants the serrated 
margin, and carries on the convex side a fine cilium. 
This variety is found on the first pair of gnathopoda. 
The tenth, eleventh, and twelfth varieties are plumose, 
and found mostly on the second pair of antennz, though 
a few are present on several other parts of the animal. 
One is short and obtuse, being crowned with numerous 
radiating cilia. It is to this variety that we understand 
Professor Hensen attributes the power of hearing. 
This great variety of form in the hairs of a single 
species is not constant. In the genus Talitrus, there is 
but a single form of hair, which is but little modified in 
the various parts of the animal. It is short, stiff, and 
blunt, and exhibits under the microscope a tendency to a 
spiral condition for about one-fourth from the extremity, 
at which distance a second but smaller process exists, so 
that the hair might be characterized as being forked, but 
for the unequal proportion of the two branches. This 
kind of hair is by no means rare in the Amphipoda. 
Those found in Orchestia, Talorchestia, Nicea, Gammarus, 
&c., are but modifications of the same form. This great 
variation in the form of the hairs is more or less common 
to all Crustacea. ‘Those in Carcinus menas have been 
