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 gether with some accessary organs, whose uses have 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 testicle, vas defcrens, 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 pc dunculated vesicle is not determined. In some species it is attached to the male organs, in others to the female, 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 au oviduct, while in others, not only the oviduct, but its external termination, may be readily 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 varied and driscriminating marks.
9. Peculiar secretions.—The molluscous animals are considered as destitute of organs for the production of 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 mucus united to a pigment whose properties have nut been sufficiently investigated.
The animals which furnish this secretion, eject it when in danger or irritated, and thus envelope themselves in a dark cloud, and elude the pursuit of their foes. A milky secretion is poured forth over the surface 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 water, have likewise the power of secreting air into those organs which contribute to their buoyancy. The Janthina vul garis 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.
to. Condition of the Mollusca.—Molluscous animals are divided, according to the situation in which they reside, into three groups, which may be termed terres 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 these, some breathe air by means of a pulmonary cavity, and come to the surface to respire. Such species fre quent the more shallow ponds and lakes. Others, re spiring by means of gills, are less dependent on the shal lowness of the water, and consequently reside in different depths.