782 



CRUSTACEA. 



dages have little or no influence upon the cur- 

 rent which is continually traversing the respi- 

 ratory antrum, and which is produced by the 

 motions of the great valvular lamina, already 

 described as belonging to the second pair of 

 the maxillipedes, and situated in the efferent 

 respiratory canal. 



The very secondary part which the flabelli- 

 form appendages of the thoracic extremities 

 play in the interior of the respiratory cavities, 

 is of itself a sure indication of the indetermi- 

 nateness of their numbers and relations to 

 the branchial pyramids. Thus whilst in the 

 Lobster and the nearly allied genera, these ap- 

 pendages, to the number of five on either side, 

 belong to the four first pairs of ambulatory ex- 

 tremities and to the third of the maxillary 

 pairs, and run from below upwards between 

 the branchial fasciculi, we only find three pairs 

 in the Brachyura, belonging exclusively to the 

 maxillary extremities, and penetrating into the 

 branchial cavities horizontally, two on the outer 

 surface of the branchiae and one between the 

 inner surface of these organs and the flancs. 



We said in beginning this article that the 

 Crustacea, by their general conformation, were 

 evidently adapted to a purely aquatic life; this 

 proposition must only be understood as gene- 

 rally applicable to the class, because there are 

 genera which form exceptions to it, in regard to 

 which we have still a few words to add. 



The Telphusiae and some other families 

 of Crustaceans have the power of emerging 

 from the water, and of entering it again 

 after a longer or shorter stay upon dry 

 land. But this fact is to be explained by the 

 smallness of the two openings by which each 

 of the branchial cavities communicates with 

 the exterior, by which means a very small 

 amount of evaporation only takes place from 

 them. The whole of the Crab tribe have, in a 

 greater or less degree, the faculty of the par- 

 ticular species mentioned, provided the air by 

 which they are surrounded is saturated with 

 moisture; because if they die asphyxiated 

 when brought into the air under ordinary cir- 

 cumstances, it is principally because their bran- 

 chiae having become dry are thereby unfitted to 

 accomplish their functions. 



But there are other species which are re- 

 markable for the faculty they possess not 

 only of living habitually out of water, but be- 

 cause they are infallibly drowned by being 

 kept long immersed in that fluid these are 

 the Gecarcini or land-crabs. Many hypotheses 

 were broached to afford an explanation of this 

 phenomenon, when a careful study of the diffe- 

 rent forms under which the organs of respira- 

 tion present themselves in these different genera, 

 led us to discover in the membrane which lines 

 the walls of the respiratory cavities, modifica- 

 tions analogous to those which are observed 

 among fishes of the family of the Acanthopterygia 

 pharyngeae labyrinthiformes, &c. Sometimes 

 we found folds and lacunae capable of servino- 

 as reservoirs of a certain quantity of water- 

 sometimes, as in the Birgns, a spongy mem- 

 brane equally well calculated to store up the 

 fluid necessary to keep the organs of respira- 



tion in the state of humidity essentially neces- 

 sary to enable them to perform their functions. 

 It is well known, too, that the Land-crabs of 

 which we are now speaking, never remove far 

 from damp situations. Some naturalists are of 

 opinion that the tegumentary membrane with 

 which the branchial cavity is invested, is also 

 the seat of active respiration; M. Geoffrey St. 

 Hilaire even goes so far as to regard the growths 

 with which the surface of this membrane is 

 covered in the Birgus, as constituting a true 

 lung. 



It would appear, consequently, that it is 

 owing to the activity of the function of aerial 

 respiration in the Gecarcini, that these ani- 

 mals are drowned when plunged under water, 

 although they be provided with branchiae ; 

 and it is owing to these organs being kept in 

 a suitable state of humidity that these creatures 

 owe, at least in part, their faculty of breathing 

 air. 



We have said above that the principal cause 

 of the death of our ordinary Crustaceans exposed 

 to the air is the drying up of their branchiae ; 

 but this is not the sole cause of the asphyxia 

 they suffer ; it would seem that the collapse of 

 the branchial lamellae which takes place when 

 these organs are not supported by the water, 

 and the greatly diminished extent of surface 

 thereby exposed to the oxygenated fluid, con- 

 tributes mainly to prevent aerial respiration 

 from proving adequate to maintain life among 

 the common aquatic Crustacea. 



With regard to the modifications presented 

 by the respiratory organs of the Onisci, which 

 like the Gecarcini live far from water, nothing 

 certain is yet known. The opinion that the 

 abdominal false limbs, which serve as respi- 

 ratory organs among the Isopoda in general, are 

 here vesicular, and perform the office of lungs 

 internally, whilst their external surface acts 

 in the manner of gills, still requires to be 

 confirmed. 



6. Of Generation. 



Sexual organs are readily demonstrated in 

 the whole class of Crustaceans, but those of 

 the two sexes never exist in the same indivi- 

 dual. The doubt which at one time pre- 

 vailed in regard to this fact, and which mainly 

 arose from no other than females of certain 

 species having ever been taken, is at once 

 put an end to by the circumstance of the con- 

 siderable dissimilarity in their external form, 

 which occurs between the males and females 

 of these species; this dissimilarity indeed is, 

 in some instances, so great that naturalists 

 were led into the error of regarding the male 

 and female of the same creature not only as be- 

 longing to different species, but even to diffe- 

 rent genera. Oviparous reproduction is also a 

 constant character of the class. 



Generally speaking, the reproductive appa- 

 ratus, whether in the male or in the female, is 

 perfectly distinct, especially at the period when 

 the organs composing it are in a state of acti- 

 vity ; and one of the most remarkable facts 

 which the careful study of this part of the 

 structure of the class has afforded, is their com- 



