ORGANS OF RESPIRATION. 537 



bution of blood-vessels, than in the macrourous species, 

 where there are about twenty-two pyramids on each side, and 

 their component lamince are subdivided into innumerable 

 small filaments, so as to present a more extended respiratory 

 surface. The laminae of each pyramid are disposed in two 

 vertical rows, with their flat surfaces superimposed; there 

 are sometimes two hundred laminae in a single row ; and 

 between the branchial pyramids of each pair of feet, a long 

 horny lamina is extended upwards, by the constant action of 

 which the currents of water are directed forwards over the 

 branchiae. In many of the brachyourous decapods, where 

 the branchial cavity and its openings are more circumscribed, 

 the currents are generally directed inwards through an infe- 

 rior orifice on each side, by the movements of a small horny 

 valvular lamina attached to the base of a maxilla, and the 

 water passes out by a superior vent, on each side of the 

 mouth, after bathing the surface of the gills. During the 

 development of the complex branchial apparatus in the 

 decapods, the highest order of Crustacea, they are observed to 

 pass through successive conditions of structure, much re- 

 sembling the various permanent forms presented by these 

 organs in the lower orders of this class. For a time, 

 they are not developed from the feet of the embryo ; next, 

 they are observed as simple rudimentary processes, of inde- 

 terminate function; later, they become distinct, vascular, 

 naked, respiratory laminae ; and lastly, they fold up under 

 the projecting carapace, and become concealed under its 

 margins. 



FOURTH SECTION. 



Respiratory Organs of the Cyclo-Gangliated or Molluscous 



Classes. 



All molluscous animals, excepting a few pulmonated gas- 

 teropods, are aquatic, and breathe by means of branchiae, 

 which present no less striking diversities of form and struc- 

 ture than are seen in most other organs of the body in this 

 most diversified subkingdom. From their aquatic and limited 

 respiration, their muscular energy is more feeble, their move- 

 ments are more slow, their whole functions more languid, 



