XXVi THE VOYAGE OF H.M.S. CHALLENGER. 



very short, as in Penseus canaliculatus, where they are subequal in length and but- 

 little longer than, the terminal joint of the peduncle ; one is cylindrical and the other 

 is flattened, and has the margins on the lower surface projecting above the middle 

 portion. In Aristeus the flagella are very unecpial in length, and very distinct in their 

 cross-section, the inner being cylindrical, slender, and longer than the animal, while the 

 outer is short and flat, with the margins thickened on the lower surface. In Solenocera the 

 flagella are subequally long and have their margins parallel, one flagellum being cylindrical 

 and the other longitudinally concave, and both truncate at their extremity ; the cylin- 

 drical is the smaller, and when at rest lies in the hollow of the other in its entire length. 



In Haliporus the flagella differ in the larger being flattened, but not fluted, and 

 both terminate in gradually tapering extremities. 



In Sergestes the secondary or inner flagellum is reduced to a small, almost rudi- 

 mentary condition, as shown in PL LXXI. fig. b; but in the male another' branch is 

 given off, which is developed somewhat like a claw or retaining hook, varying in shape 

 in different species. As this is only present in the male, it must be of value in its 

 relation to sex, and must be of more importance than its simple character would seem 

 to suggest. 



According to my observation, in the specimens of this collection the membranous 

 cilia, or sensory rods, are less numerous and less important among the Dendrobranchiata 

 than in the other divisions. 



In the Phyllobranchiata the first pair of antennae is developed upon the same 

 general plan as in the two preceding divisions ; but it is flattened out and cupped to 

 receive the ophthalmopoda. The prosartema is never present, but on the outer side the 

 stylocerite is developed into a large, flattened plate, generally sharp-pointed, but some- 

 times, as in Pandalus, rounded instead of being styliform. 



The acoustic apparatus in Crustacea has been extensively studied. Dr. von Hensen, 

 in his memoir on the subject, 1 has described it in twenty-eight species, but that is a 

 small number compared with those that have not been examined. The direction of 

 the research appears to show that in the Trichobranchiata particles of sand take the 

 place of otoliths, whereas in the Dendrobranchiata and Phyllobranchiata the latter 

 are more constant, and certainly in a higher degree of development, as may be seen 

 in the genera Tozeuma and Anchistia, in both of which the otolith is as well formed as 

 in any of the Schizopoda or Sergestidse. 



Among the Macrura generally the first pair of antennae terminates in two flagella, 

 and, so far as my experience enables me to say, Lucifer is the only genus in which it 

 terminates in a single flagellum (PL LXXIX. fig. lb). 



The outer flagellum supports a number of membranous organs, which are generally 

 massed together at the base, and are more abundant in the male than in the female ; the 



1 Zeitschr.f. wiss. Zool., Bd. xiii., 1863, pp. 319-412 (p. 18 sep. copy). 



