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



uses its larger hand, by holding its food while the smaller carries it to the mouth. We 

 may suppose that the blind Thaumastocheles, resting upon a bed of Globigerina or 

 Diatom ooze, can, by raking the surface of the mud, fill the long comb-like hand with 

 multitudes of minute animals more or less adapted for its food ; that then the smaller 

 hands gather up the larger and more suitable portions, and carry them to the mouth. 

 It is interesting to learn that a near ally of TJiaumastocheles zaleuca appears to have 

 existed so far back as the Cretaceous formation, in the species Stenocheles esocinus of 

 Fritsch and Kafka j 1 but whether this was also a blind animal or not I do not 

 know. 



In the Dendrobranchiata the first is the smallest of the three chelate pairs character- 

 istic of the division, one of which is large ; the first is moreover the shortest, and 

 appears from its relative length to be capable of reaching the mouth. 



It is also noticeable in this division, that in those genera in which there is a 

 downward tendency from the normal form and power, the change takes place at each 

 extremity of the pereion. The first pereiopod is the first to become enfeebled, as may be 

 seen in the Sergestidse, in which the first has lost its chelate structure, and yet retains a 

 grasping or holding power in the peculiar adaptation of the carpo-propodal articulation ; 

 while at the posterior extremity of the pereion the fifth pair has become little more than 

 rudimentary, and the fourth is much diminished in importance and value. 



In the Phyllobranchiata the first pair of pereiopoda varies greatly in relative form 

 and size, being sometimes the largest, as in Alphevs, but more commonly very much 

 the smallest, as in the Palsemonidse ; again, as in the Crangonidse, it is reduced to a 

 subchelate condition, in consequence of the pollieiform angle of the propodos being 

 reduced to a small tooth-like point, as in Crangon and its immediate congeners. In 

 Glyphocrangon this point is altogether absent, and in Nika the change in structure differs 

 on the two sides, this pair being simple on one side and chelate on the other. In the 

 Pandalidse it exists as a pair of simple pointed legs, styliform in appearance. 



In the freshwater genus Atya this pair of pereiopoda is developed into a kind of 

 brush to provide the mouth with supplies of fine mud on which the animal lives. The 

 extremity of both the finger and thumb is provided with a tuft of long bristles, 

 which, when the hand is open, form a kind of fan which retains the fine mud ; when 

 the hand is closed, the bristles are closed around the mud, compressing it into a pellet, 

 which is passed into the mouth with great rapidity. 2 



The Second Pereiopoda. — The second pair of pereiopoda varies much in some 

 families and but little in others. All through the Trichobranchiata it is chelate and 

 only of moderate proportions, being much smaller than the first and as large as or larger 



1 Die Crustaceen der Bohmischen Kreideformation, 1887. 

 - Fritz Miiller, Kosmos, Bd. viii. p. 117, 1881. 



