780 



CRUSTACEA. 



Fig. 420. 



Fig. 428. 



Branchial cavity of 

 the Maja Squinado 

 laid open. 



a, branchiae ; b, vault of the flancs ; c, carapace \ 

 d, efferent duct ; e, valve. 



the carapace. This last part of the walls of the 

 branchial cavity presents an epidermic layer of 

 extreme thinness, but covering a thick and 

 shaggy membrane, the texture of which is 

 found to vary, as we shall see by-and-by. 



The cavity thus formed communicates ex- 

 ternally by two passages, the one destined for 

 the entrance, the other for the exit of the water 

 necessary to respiration. The disposition of 

 the efferent opening varies but little; that of 

 the afferent orifice, on the contrary, presents 

 great varieties in the different groups of which 

 the class of Decapods is composed. 



The efferent orifice always occupies the ante- 

 rior extremity of the branchial cavity, and is 

 continuous with a canal (rf, Jig, 426 and f,fig. 

 428) the parietes of which are formed su- 

 periorly by the epimeral pieces of the last ce- 

 phalic rings, and inferiorly by the pterygo- 

 stomian portions of the carapace (6, Jig. 427;. 



Fig. 427. 



Head of the Maja Squinado. 



a, afferent opening of the branchial cavity , b, 

 carapace ; c, anterior extremities ; d, posterior 

 maxillipedes. 



This canal runs forwards, passes to the out- 

 side of the oral apparatus, and terminates in 

 front of the mouth (gjig. 428). In its interior 

 there is a large valve, which is falling and rising 

 continually, as if it moved upon a pivot, and 

 which in this way occasions a rapid current 



The same parts, the posterior maxillipedes and a por- 

 tion of the carapace having been removed. 



a, afferent opening ; d, portion of the posterior 

 maxillipedes ; e, commencement of the efferent 

 canal (f)g, the termination of the efferent 

 canal ; h, the valve. 



from behind forwards in the water with which 

 the cavity is filled. This valvular apparatus is 

 neither more nor less than the flabelliform 

 appendage of the second pair of maxillipedes 

 which acquire dimensions in relation with the 

 importance of the new function they have here 

 to perform (A, jig. 428). 



In the long-tailed Decapoda, and in the 

 greater number of Anomoura of the same 

 family, the respiratory cavity is open along 

 the whole extent of its inferior edge; the 

 carapace is not applied accurately to the 

 lower margin of the vault of the flanc, and 

 it is by the empty space thus left above the 

 base of all the extremities that the water makes 

 its way to the branchiae. In the Brachyura 

 the afferent orifice of the branchial cavity is 

 more circumscribed, but varies in a still greatex* 

 degree. In nearly all the Crustacea it exists 

 almost immediately in front of the base of the 

 first pair of ambulatory extremities, and con- 

 sists of a kind of cleft, of considerable breadth, 

 which in this place occurs between the edge of 

 the carapace and the thorax (a, jig. 427), and 

 which is occupied by a prolongation of the ba- 

 silar joint of the external maxillary limb (d) y 

 disposed in such a manner as to close it com- 

 pletely or to open it at the desire of the ani- 

 mal. In the genus Dorippus a slight variety 

 in the disposition of this opening is ob- 

 served ; here at first view it appears to be 

 pierced directly in the pterygostomian portion 

 of the carapace; but it is in reality formed 

 by an empty space left between the edge of 

 the dorsal shield and the base of the external 

 maxillary limb; only here, this space, in- 

 stead of presenting itself immediately in front 

 of the base of the anterior extremities, is se- 

 parated from this by a prolongation of the 

 carapace. In the genus Ranina the carapace 

 is joined to the thorax above the whole of 

 these limbs, so as to leave no opening in this 

 situation for the passage of the water, and it 

 is at the origin of the abdomen that the afferent 

 opening of the branchial cavity occurs. Lastly, 

 in the Leucosia, this cavity is in like manner 

 completely closed above the base of the extre- 

 mities, and it is by a conduit parallel to the 

 efferent canal, and opening outwardly likewise 



