604 



MOLLUSCA. 



Mollusca. somewhat laterally, and communicates externally by 

 "Y"""' an aperture which the animal can open or shut at plea- 

 sure. 



The mollusca which breath by means of gills exhibit 

 very remarkable differences, in their number, structure, 

 and position. In some cases there is a single cavity 

 communicating by an aperture through which the wa- 

 ter enters. The walls of this cavity exhibit an uneven 

 surface, disposed in ridges, which are the gills, and on 

 which the pulmonic artery is expanded. This struc- 

 ture exhibits itself in the Gasteropoda pectini-bran- 

 chia. 



In many cases the gills, though seated in a cavity, 

 like the former, and equally exposed to the contact of 

 the surrounding element, are two in number, one on 

 each side, as in the Cephalopoda. In the Conchifera 

 they are four in number, two on each side like leaves, 

 and extending the whole length of the body. In these 

 the water is admitted at the pleasure of the animal. 



The gills of other mollusca are seated internally, and 

 consist either of arborescent productions, or simple cu- 

 ticular elongations, within which the pulmonary artery 

 terminates. In some of these, as the Pteropoda, the 

 branchial surface is constantly exposed to the action of 

 the surrounding water ; while in others, as the Gastero- 

 poda, midibranchia, and teclibranchia, the cuticular ex- 

 pansions, which are analogous to gills, are retractile at 

 the will of the animal. 



By means of the characters furnished by the circulat- 

 ing and" respiratory systems, the molluscous animals 

 may be divided into several distinct classes. But as we 

 shall employ these characters in the construction of the 

 different divisions to be employed, it is unnecessary, 

 in this place, to enter into their details. 



He rodu **' -R e P r ductive System. The animals now under 



" consideration present nearly all the modifications of 



11 V I. r>> alLIll . i i i i i FT*I 



generation which organized bodies exhibit. They are 

 not indeed known to produce their young like buds, as 

 in the fresh- water polypus (Hydra) nor to multiply by 

 the spontaneous division of individuals, as some Nereides 

 and Planaris. But the three following modes of repro- 

 duction have been detected. 



In the first the sexes are distinct, as in the higher 

 classes of animals. This mode is exhibited in the Ce- 

 phalopoda and some Gasteropoda. In the former, how- 

 ever, there is no union of the sexes previous to fecunda- 

 tion. The eggs are first excluded by the female, and 

 the" male afterwards covers them with the impreg- 

 nating fluid, after the manner of fishes. In the Gaste- 

 ropoda, with the sexes distinct, a union takes place by 

 which the eggs are fecundated. After impregnation, 

 the eggs are either excluded previous to the young issu- 

 ing from them, or retained in the body of the animal 

 until they are hatched. 



In the second the sexual organs are distinct, but 

 they both occur in the same individual ; here, however, 

 a union of two individuals is necessary, during which 

 each impregnates and is impregnated. The snail and 

 slug may be quoted as examples. 



In the third mode of generation, which may be re- 

 garded as the most complete hermaphroditism, there is 

 neither a difference of sex nor an obvious difference of 

 sexual organs. All that can be detected, is connected 

 with the female parts, and no union of individuals is 

 requisite. This is the mode of generation exhibited by 

 the Gasteropoda scuti-branchia and cyclo-branchia by 

 the whole of the Conchifera and Tunicata. 



In the first division of the first tribe, the female pos- 

 sesses an ovarium and oviduct, and the male a testicle, 



vas deferens, seminal bag, prostate gland, and penis, to- Mollusca. 

 gether with some accessory organs whose uses have '"""V" 

 not been ascertained. In the second division of the 

 first tribe, there is in the female an ovarium oviduct 

 and uterus, and in the male a testicle, vas deferens, and 

 penis. 



In the second division, the female organs consist of 

 an ovarium oviduct and uterus, and the male of a testi- 

 cle vas deferens, pedunculated vesicle and penis. These 

 organs, all occurring in the same individual, have their 

 openings in what is termed the common cavity of ge- 

 neration which opens externally. The use of the pe 

 dunculated vesicle is not determined. In some species 

 it is attached to the male organs, in others to the fe- 

 male, or to the common cavity. 



In the last division, the ovarium is the principal 

 and only organ of generation. In some species it is dif- 

 ficult to discover an oviduct, while in others, not only 

 the oviduct but its external termination may be readi- 

 ly traced. 



The organs of generation furnish many important 

 characters for classification. The external openings are 

 those which are detected with the greatest facility,, but 

 the structure of the internal organs exhibits more va- 

 ried and discriminating marks. 



9. Peculiar secretions. The molluscous animals are Peculiar 

 considered as destitute of organs for the production of secretions. 

 urine, but they possess various organs for the secretion 



of peculiar fluids or solids, some of which are useful in 

 the arts. 



The coloured fluid, which is secreted by the Cepha- 

 lopoda and some of the aquatic gasteropoda, appears to 

 consist chiefly of a peculiar mucous united to a pig- 

 ment whose properties have not been sufficiently inves- 

 tigated. The animals which furnish this secretion eject 

 it when in danger or irritated, and thus envelope them- 

 selves in a dark cloud, and elude the pursuit of their 

 foes. A milky secretion is poure<l forth over the sur- 

 face of the skin of some slugs when irritated. Other 

 coloured secretions may likewise be detected in the 

 mollusca, to which we shall afterwards advert. 



The threadlike secretions, termed a byssus, with which 

 some molluscous animals, especially among the Conchi- 

 fera, fix themselves to other bodies, appear to be of an 

 albuminous nature. A few species in this division have 

 the power of secreting a luminous fluid, which phospho- 

 resces or shines in the dark. Its nature, and the organs 

 in which it is elaborated, have not been investigated. 

 It is probable that some animals, as those which have 

 the faculty of raising or lowering themselves in the wa- 

 ter, have likewise the power of secreting air into those 

 organs which contribute to their buoyancy. The Jan- 

 thina vulgaris may here be quoted as an example. 



Morbid secretions likewise occur among the animals 

 of this division, chiefly, however, among the Conchifera. 

 The most important of these are pearls, so much prized 

 as ornaments of dress. 



10. Condition of the Mollusca. Molluscous animals Condition 

 are divided, according to the situation in which they of the raol- 

 reside, into three groups, which may be termed terres- l 



trial, fluviatile, and marine. Those that inhabit the 

 land, belong exclusively to the gasteropoda. Among 

 these some prefer open pastures, others the rubbish of 

 old walls, while not a few reside in woods or among 

 dead leaves and putrid plants. All the animals of this 

 group respire by means of a pulmonary cavity. 



The fluviatile mollusca, or such as reside in fresh wa- 

 ters, include not only many gasteropodous genera, but 

 likewise some belonging to the Conchifera. Among 



