A BOX OF BKANCHLSE 369 



CHAPTER XXIII 



TKIBE in. VALVIFERA 



HEEE the uropocls undergo a remarkable metamorphosis, 

 and assume a function distinct from any that they have 

 elsewhere, for like a pair of folding doors they form a great 

 part of the ventral surface of the pleon, these valves closing 

 over the five pairs of branchial pleopods or opening to 

 admit the water to them. 



In the ' History of the British Sessile-eyed Crustacea,' 

 vol. ii. pp. 358, 368, 375, 378, the uropods are successively 

 spoken of as ' the first or anterior pair of pleopoda,' as per- 

 taining to ' the first segment of the tail,' as being absent in 

 the Idoteidas, where there is said to be a ' conversion of 

 the fifth pair of pleopoda into a continuous operculum for 

 the protection of the branchial organs,' and lastly, in the 

 genus Idotea, as c a strong outer pair (which are the ter- 

 minal uropoda), forming an opet'culum ' over the ' five 

 pairs of very delicate branchial appendages.' Of these 

 four statements the first two are consistent but erroneous, 

 the last two are inconsistent with the first and with one 

 another ; only the final one is correct. All the four, as it 

 happens, appeared in the same number of the \vork, namely, 

 Part 19, published October 1, 1867, so that the confusion 

 is difficult to account for, even as an accident of dual 

 authorship. 



The tribe includes two well-separated families, the 

 Arcturidas and Idoteidas. 



Family 1 . Arcturidce. 



The form of the animal is elongate and sometimes 

 cylindrical ; the segments of the pleon are more or less 



B B 



