M O L L U S C A. 



605 



Moiiusci. these, tome breathe air by means of a pulmonary cavi- 

 ~-^, ' ty, and come to the surface to respire. Such species 

 frequent the more shallow ponds and lakes. Others, 

 respiring by means of gills, are less dependent on the 

 shallowness of the water, and consequently reside in 

 different depths. 



The marine mollusca include genera of all the classes. 

 Some, as many of the Cirrhipoda,' instead of living in 

 the water, prefer a situation where they may only be 

 bathed occasionally with the flood tide. Others bur- 

 row in the sand or adhere to the rocks which are left 

 dry by the receding tide. These are termed littoral 

 species. Many, however, which have been denomina- 

 ted pelagic, reside in the deep, and are seldom obtain- 

 ed but by dredging, or when thrown ashore during 

 (terms. 



The effect of temperature in regulating the distribu- 

 tion of molluscous animals has not been investigated 

 with any degree of care or success. Over the terrestrial 

 and fluviatile species it probably exercises a very power- 

 ful controul, greatly limiting their geographical range. 

 In proof of this, it may be stated that the south of France 

 poeiesMi several specie* not to be found in Eng- 

 land, while in England there are a few which have not 

 been detected in Scotland. But among the marine 

 mollusca, the influence of climate is not felt in the same 

 degree. Living in an element whose bulk and motions 

 guard it equally from the extremes of heat or cold, 

 these animals, like the sea-weeds, have a very exten- 

 sive latitudinal and longitudinal range. Thus, some 

 are common to Greenland and the Mediterranean, 

 others to Britain and the West Indies. The mollusca 

 of the tropical seal, however, differ widely as a whole 

 from those of the temperate regions. Some of the 

 forms appear to be peculiar to warm regions, and in 

 general the intensity of colour decreases as we approach 

 the poles. But as there have been few cultivators of 

 this branch of science, the geographical distribution of 

 the species has been but imperfectly explored. How 

 tew parts of either England or Scotland have been sur- 

 veyed by the eye of the helmonthologist, so that many 

 pecies whose range is considered as limited, may ere 

 long be found to be extensive. 



If the observations are few and imperfect, which 

 have been made on the influence of temperature in re- 

 gulating the physical distribution of mollusca, we are 

 still in greater ignorance with regard to the power of 

 habit. In the fleet* rocks, the relics of marine and 

 fluviatile mollusca, are found mixed in the same bed. 

 This circumstance gave rise to the inquiry, how far the 

 mollusca of fresh water can be habituated to sea water 

 and woe eerta. In the account of the proceedings of 

 the National Institute of France for the year Is 1C', we 

 are informed that M. Beuchant, Professor at Marseilles, 

 has directed his attention to this subject. He found, 

 that all these animals die immediately if we suddenly 

 change their place of abode ; but that, if we gradually 

 increase the proportion of salt in the water for the one 

 set, and dimmish it for the other set, we can, in gene- 

 ral, accustom them to live in a water which is not na- 

 tural to them. He found, however, some species which 

 resisted these attempts, and which could not bear any 

 alteration in the quality of the water in which they re- 

 side. Before much confidence can be placed in the ac- 

 curacy of these results, it would be desirable that the 

 experiments were repeated by other observers. There 

 are, indeed, many sources of error to be guarded 

 against. When we change animals from fresh to salt 

 water, or from salt water to fresh, we must necessarily 



derange their motions by compelling them to reside in Molluscs. 

 a medium of a different degree of density from the one "* """" 

 which they have been accustomed to dwell in, and to 

 which the arrangement of the different parts of the 

 body is adapted. By such a change of place, it would 

 be difficult for those which breathe air to come to the 

 surface and descend again in their new situation. In 

 those with gills, the application of a new kind of fluid 

 to the surface of such delicate organs would consider- 

 ably influence the function of respiration. The change 

 of situation would likewise be accompanied by a corre- 

 sponding change of food, and consequently, not merely 

 the organs of locomotion and respiration, but likewise 

 those of digestion would suffer a derangement in their 

 operations. We know that the power of suffering in 

 the animals of this class is very great, and that they 

 survive, though sadly mutilated. Some of the snails 

 will live in a quiescent state for years, without food, 

 and almost without air. Unless, therefore, the animals 

 subjected to these experiments of a change of situation 

 have been observed to grow on the food which it spon- 

 taneously yields, to execute their accustomed motions, 

 and, above all, to propagate their kind, we shall be dis- 

 posed to conclude, that patient suffering has been mis- 

 taken for health, and vivadousness for the power of ac- 

 commodation. 



As connected with the physical distribution of mol- 

 luscous animals, we may here offer a few observations 

 on the revolutions which they have experienced. From 

 the oldest secondary rocks to the newest alluvial depo- 

 sits, the remains of the hard parts of these animals may 

 be observed. In the newer situations, these parts are 

 less altered than in the older ones, in which they are 

 frequently changed into calcareous spar, clay, flint, or 

 pyrites. The forms exhibited by the remains in the 

 older rocks are different from those in the newer ones, 

 and intimate that they have belonged to races now no 

 longer existing on the surface of the globe. These re- 

 mains do not characterize any particular beds or for- 

 mations, different beds often producing similar re- 

 mains, and similar beds containing dissimilar fossils, in- 

 timating that the physical and geographical distribu- 

 tions of these animals in the earlier parts of the earth's 

 existence were regulated nearly by the same laws 

 which prevail at present. But into this interesting sub- 

 ject it is impossible at present to enter. 



The molluscous animals furnish an agreeable repast 

 to many quadrupeds, birds, and fishes. To man they 

 yield a great deal of palatable and nutricious food. 

 Those which inhabit the sea are held in the highest es- 

 timation, while the terrestrial and fluviatile races are 

 generally neglected. 



The manner of preparing and preserving the hard 

 parts of the mollusca has been already given in suffi- 

 cient detail in the conclusion of our article CONCHOLO- 

 oy, to which we refer the reader. There is only one 

 method of preserving the softer parts immersion in 

 spirits of wine. But in the execution of this plan some 

 caution is requisite, otherwise the object will appear a 

 shrivelled shapeless mass. The animal should be per- 

 mitted to die slowly, that the different parts may be- 

 come relaxed, otherwise the examination of the form of 

 the body at a future period becomes impracticable. A 

 quantity of the spirits ought to be injected into the 

 stomach or other cavities of the body immediately after 

 death, to prevent putrefaction, as it frequently happens, 

 when the body is immersed in spirits without such pre- 

 caution, that the viscera become unfit for examination 

 while the integuments remain sound. 



