INTRODUCTION. xxiil 



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 011 the first pair of gnathopoda. 



The tenth, eleventh, and twelfth varieties are plumose, 

 and found mostly on the second pair of antennae, 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 mcenas have been 



