MALACOLOGY. 



sumes the shape of a sucker, as in the Nucula ; a lingual 

 appearance, as in the mussel ; in the Venus, the form 

 of an arc ; and in the Ckama, the form of the human 

 foot. Other muscular portions of the animal's body, 

 in conjunction with that termed the foot, occasion 

 by their attachment to the shells of bivalves what is 

 called the muscular impressions, serving, when well 

 defined, to guide the malacologist with considerable 

 truth in determining the order of the animal they 

 enclose. In the locomotive system of bivalve muscles, 

 their mode of articulation, by means of an external, 

 internal (and in some species both combined), liga- 

 ment, that part must be examined with particular 

 attention, as aiding or acting as an antagonist to the 

 adductor muscles. It is formed of transverse horny 

 fibres, which in their natural position connect the 

 valves, but constantly tend to keep them open by 

 their elastic nature, like a spring, occasioning the 

 animal no trouble in opening its shell, and not re- 

 quiring the body to be provided with another set of 

 muscles for that purpose. This is a simple and 

 beautiful provision of nature ; the general wants of 

 the animal requiring its shell to be more frequently 

 open than closed. 



A very singular fact, observable in the tendinous 

 bundle of muscles forming the foot, is that some spe- 

 cies possess the faculty of becoming agglutinated and 

 attached to submarine bodies, serving as an external 

 point of resistance or repose. This constitutes what 

 is termed the byssus, that substance not being, as 

 many authors supposed, a filamentous excretion, con- 

 tained in a given gland, and spun out by the animal's 

 foot, but it is a continuation of the fibrous muscles, one 

 portion of which is living in the animal and possess- 

 ing contractile powers, while the other, attached to 

 the object serving to fix it to that spot, has become 

 devoid of sensation, and incapable of feeling the injury 

 it would otherwise have sustained by collision with 

 different bodies, had its sensibility been preserved 

 externally a.s it is internally. Another extraordinary 

 provision of nature which we cannot here pass over 

 unnoticed, which, however, wants the positive con- 

 firmation of the preceding one, is the apparent pro- 

 bability that the animal possesses the faculty of 

 changing the position of its adductor muscles, in pro- 

 portion as it increases in size ; for if, in a shell not 

 having attained its full period of maturity, this muscle 

 was centrical, it must, to retain that position in the 

 animal's body, move backward in its points of adhe- 

 rence when it attains a greater growth. This is illus- 

 trated in many bivalves ; we will instance the most 

 easily to be examined the oyster, in which the mus- 

 cular impression indicates a change of position very 

 distinctly marked of the subcentral muscle, not that 

 it need be detached all at once to effect this, but 

 probably because an anterior arrangement of muscu- 

 lar fibres become detached as a posterior set are pro- 

 duced -, the use of the one being no longer necessary, 

 or kept in the activity of health, lose their energy 

 and as it were die off. No certainty is yet acquired 

 on ihis head, but it has given rise to much speculation, 

 and various extremely ingenious opinions. 



In the Lepadicea, no locomotive organs, properly 

 so called, can well be admitted, they live and die on 

 the spot which gave them birth ; and the Chiton, in 

 its locomotive parts, approaches very near to the 

 true mollusc, tlie whole of the lower part of its body 

 being occupied by a species of foot, very analogous 

 to that of the Patella, &c., while its shell possesses 



as muny double pairs of oblique muscles placed on 

 the right and left sides as there are testaceous por- 

 tions. The powers of locomotion depending always 

 upon the necessity of existence under the various 

 positions of animal life, it is evident, with regard to 

 molluscs, that it must be extremely restricted in most, 

 and even not necessary at all to many. Whatever 

 creature is gifted with the greater number of senses, 

 it also possesses the better organised muscular system 

 to exercise the purpose of those faculties ; thus, the 

 Sepia roams'at large in pursuit of that first of all neces- 

 sities food, while the Axcidia, possessing fewer of 

 the senses, remains permanently fixed to submerged 

 bodies. We may, as it were, sum up the nature of 

 locomotive evidences in molluscs, and give a few 

 conclusions drawn from such facts as have fallen 

 under the naturalist's notice, incomplete and in many 

 instances uncertain, resting only upon analogous 

 reasoning, but satisfactory as far as such can be. 

 Various kinds of locomotion exist in mollusca ; some 

 direct their movements by means of symmetrically 

 placed appendages acting as oars, using them much 

 in the same way as fishes employ the fins, and these 

 organs sometimes enable them even to spring into 

 the air to a considerable height, which is witnessed 

 the Cabiiar ; a similar operation is sard to be 

 effected by some species of bivalves, whose expanded 

 valves are employed as points of resistance in the 

 water. Another kind of natation is occasioned by a 

 very compressed foot alternately moving from right 

 to left, as it is seen in the Carmaria, but this always 

 takes place in a reversed position, that is, with the 

 back beneath and the belly uppermost. A third, yet 



Carinaria the shell of its natural size and a reduced figure of 

 the animal with the position of the shell on it. 



more singular, has been observed in the Sepia, Calmnr, 

 &c. This is performed by a sudden contraction of 

 the envelope of the animal's body, by which the fluid 

 contained at the moment of its complete extension is 

 expelled, or spirted out, giving an impetus which is 

 directed by means of the muscular appendages of the 

 body in the direction required. Some molluscs float 

 on the surface of the water, propelled as chance may 

 direct by the caprice of wind or waves, supported by 

 means of a vesicular apparatus, as in the lanthina ; 

 and the Argonauta is elegantly described as having 

 an inhabitant which navigates its frail bark by means 

 of an extended portion of its mantle, and guided in 

 its course by oar-like muscular appendages ; a 

 reference to that article will more fully explain this 

 assertion, and the other still contradictory opinions 

 respecting that mollusc, and the shell it inhabits. 



