38 



common Entomostraca, such as the Cyclops, Daphnia, Cypris, &c, 

 which are so abundant in pools of stagnant water, may be cited as ex- 

 amples of this class. 



In the second stage we find no proper branchiae, but we have cer- 

 tain appendages to the digestive or locomotive systems on which are 

 developed flattened membranous vesicles, highly vascular, which an- 

 swer the purpose of branchiae, and we have examples in the genus 

 GammaruSy Talitra, &c, of this form of respiratory apparatus. 



In the third stage we have the best examples in the Squilla, (Plate 

 iii. fig. 1). Proper branchiae are developed, but they are not contained 

 in any cavity, but are attached to the basilar joints of the first five 

 pairs of abdominal extremities, and each consists of a long cylindrical 

 tube, from one of the sides of which proceeds a series of small tubes, 

 disposed parallel to each other, and not unaptly likened by Milne 

 Edwards to the pipes of an organ ; these support, in their turn, a 

 series of long, cylindrical, and very numerous tubes, (PI. iii, figs. 2, 3). 

 It may further be remarked that these branchiae are continually ex- 

 posed to the action of the surrounding water. 



Lastly, the highest state of development of the branchial apparatus 

 is found in the common decapod Crustacea, such as our ordinary 

 crabs, (Plate iii, fig. 4), and lobsters, (Plate iv. fig. 1.) Not only have 

 we in them a special apparatus for performing the function of respira- 

 tion, but we have it lodged in an especial cavity, and certain accessary 

 organs developed in the form of flabella, or sweepers, by means of 

 which it has been supposed that currents of water, necessary for the 

 decarbonization of the blood, are made to pass and repass over the 

 whole of the branchial apparatus, (Plate iii., figs. 4, 4a, Ab). 



In the lobster, the branchial cavity is open the whole way along its 

 inferior edge, but in the crabs the afferent and efferent orifices are 

 more circumscribed. 



In the crab, the branchiae themselves are of the shape of a quad- 

 rangular pyramid (Plate iii. fig. 7), the base being attached to the low- 

 er part of the chamber, or to a membrane which extends from its 

 inferior edge to the basilar articulation of the corresponding limb. 

 Each branchia consists of a septum, on each side of which are deve- 

 loped numerous lamellae or plates; these lamellae are arranged at 

 right angles to the septum, and are of a quadrangular figure. On the 

 upper side is developed a small hook or spine, to which attention 

 will be again presently directed, (Plate iii. figs. 8, 3a, 10a). Upon the 

 flattened surfaces of these plates the pulmonary capillaries are distri- 

 buted, and the blood is returned after its purification to the heart by 



