41 



one on either side of the septum, there are two rows of spines on 

 each branchia, (Plate iii. figs. 8 — 10, a, a). In some of the small crabs, 

 spines are developed on the sides of the lamellae as well as on their 

 broad surfaces. The spines are of much firmer consistence than any 

 other part of the lamellae, and when touched with a pointed instru- 

 ment the lamellae can be moved backwards and forwards, and in pass- 

 ing the instrument from one end of the branchia to the other, a motion 

 is produced like to that when a finger is passed along the tops of the 

 teeth of a comb, the branchial lamellae are rapidly separated one from 

 the other ; and this led to the conclusion that by the motions of the 

 flabella the spines on the branchiae may in some way become entan- 

 gled with those on the flabella, and the branchial laminae then would 

 be separated, and the water contained in the branchial chamber 

 would by this means be brought into better contact with the pulmo- 

 nary capillaries distributed on the lamellae. 



When we reflect on this subject, some such contrivance as I have 

 described must be necessary, and the very structure of the branchiae 

 themselves would warrant such a conclusion. The branchiae, consist- 

 ing as they do, of many hundreds of little plates closely packed to- 

 gether, would have no chance of having the blood circulating on their 

 sides brought into contact with the water, unless there were some 

 means of separating them one from the other. The Squilla and some 

 of the other lower forms of Crustacea do not require such an appara- 

 tus, for every part of their rudimental branchial apparatus is con- 

 stantly bathed in fluid, and being in continual movement, every part, 

 however minute, must be in contact with water, they therefore require 

 no sweeping organs; but the case is altered when we come to those 

 higher Crustacea where the branchiae are contained in a cavity, and 

 through which cavity, even in the progression of the animal, no water 

 would flow unless the valvular laminae belonging to the second pair of 

 legs were in constant movement. 



In the lobster tribe it has been stated that we have a differently 

 formed branchial apparatus to what occurs in the crabs, consisting of 

 a number of cylindrical tubes attached by one extremity to a central 

 stalk, (Plate iv. figs. 2, 3, 4, 5, 6). The free ends of these tubes are 

 rounded, and within them the blood-vessels run. Fig. 8 represents 

 one of the tubes highly magnified. If the flabella in these animals 

 were constructed in the same way as those of the crabs, the presence 

 of a spine would be required on each tube, but such is not the case, 

 the flabella are arranged on a different plan to overcome this diffi- 

 culty ; instead of being thin and provided with hairs only on the sides, 



