ON MYSIS. 15 



of this limb, and fully exposed to the action of the 

 ambient fluid : in the other Crustacea to which they most 

 nearly approach in appearance, as the Shrimps, Prawns, 

 and Lobsters, it may be observed, that there is a single 

 TOW oifive feet on each side, (and they are hence designated 

 by the family title of Decapoda) and these adapted to 

 crawling, except some of the anterior pairs, which are 

 generally chelate or formed into a kind of pincers ; and 

 further, that the branchia or gills, which are attached to 

 the outer part of their base, are reflected backwards and 

 upwards, and entirely protected and concealed by the sides 

 of the shell or cephalo-thoracic-clypeus. 



The number of feet in this Genus, and in such of the 

 Shizopoda as are thoroughly known, is eight in each of the 

 four rows, in all thirty-two feet ! of which sixteen are adap- 

 ted to prehension, and sixteen to swimming. We plainly 

 perceive in this instance, how organization modifies the 

 habits of these two descriptions of Crustacea, the Shizopoda 

 being almost always found swimming near the surface of 

 the water, whilst the Decapoda with extended tails, 

 (Macroura) as the true Shrimps. &c. are obliged to con- 

 fine their perambulations to the bottom ; these latter it is 

 true are not altogether incapable of swimming, but when 

 they do, it is evidently an efibrt, and eifected solely by 

 means of the subcaudal fins : it results from the above 

 economy, that these, clear the bottom of numerous impu- 

 rities, and by their predacious habits, keep in check the 

 mollusca,annelides,&c. below, whilst those, (theShizopoda) 

 eflect the same purpose in the supernatant element, where 

 the medusae and lighter portions of extraneous matter, 

 furnish an equally abundant stock of nutriment ; thus the 

 ocean is freed from much of its impurity, and the balance 

 of nature sustained. 



Confining our views to the Genus Mysis or Opossum 

 Shrimp, it may be observed by reference to the magnified 

 figure, Plate I. fig. 1. that its general appearance approx- 



