The circulating system, which, as has been stated, is complete and double in all the Mollusks, is provided in all the species with a systemic heart, and in the highest organized species with a distinct heart for the lesser or branchial circulation. The systemic heart that appears in the sessile Tunicaries as a vasiform undivided ventricle, which, however, is in closed in a distinct pericardium. It is concen trated into a more compact muscular organ in the Con'cliffers, and divided into a venous and arterial chamber; but the auricle, and some times also the ventricle, as in Arca, is sub divided and segregated, according to the law of self-repetition, which is exemplified in all the systems of organs at their first appearance in the animal kingdom. The heart of the Gastropods exhibits the highertype of a single auricle and ventricle, both placed at the termi nation of the lesser and the commencement of the greater circulation. In the highest Cepha lopods the course of the blood is accelerated in both circulations by a muscular ventricle, but the superadded analogue of the pulmonic heart here, likewise, at its first appearance illustrates, by its separation into two distinct ventricles, the law above alluded to in reference to the systemic heart of the lower Mollusks.
The respiratory organ is distinctly developed in all Mollusks, and is subject to the greatest variety of forms in this division of the animal kingdom; yet, with the exception of the small sessile order of Ascidians at the lowest extreme, connecting the Mollusks with the Zoophytes, it affords perhaps the best positive character of this great primary group of the animal king dom; for whereas, in the Articulate division, the breathing organs are lateral or open upon or towards the sides of the body, and in the Vertebrate division communicate with the oral extremity of the nutritive canal, in the Mollusca they are connected with the anal outlet.
The uropoietic system, where traces of it are recognizable, as in most Gastropods and in Cephalopods, likewise communicates with the respiratory cavity.
The rich endowment of vibratile cilia de serves to be noticed as characterizing the bran chial organs, and constituting the chief mecha nism of respiration in most Mollusca. The organs of vegetative life subservient to the pro creation of the species are not less remarkable for bulk, variety, and complexity than are those which minister to the preservation and growth of the individual. Although comparatively simple and reduced to the essential formative organs in the Acephala, they are, with very few exceptions, placed in distinct individuals, that is to say, one Ascidian or Oyster possesses only the testicle, and is a male ; another only the ovarium, and is a female. In the order Conchifera, the females have the gills modified to serve as a receptacle for the impregnated ova during foetal development.
Among the Encephalous Mollusks are many hermaphroditical species, some with the male and female organs terminating sufficiently close together for independent or self-impregnation; others having the outlets of the two organs remote, and requiring the concourse of another individual in reciprocal fecundation ; or the same individual is impregnated by another and impregnates a third, as is curiously exempli fied in the nuptial chain thus formed by nu merous individuals of the Marsh-snails ( Lyn nwa). The Trachelipods and Cephalopods are again dicecious, like the lowest classes of Mollusks, but exhibit the generative organs under the highest stage of complication; and some of the latter class are remarkable for the expulsion not only of the ova aggregated in groups contained in special receptacles, but also of the spermatozoa in a similar state of aggregation in cylindrical cases, which, by the arrangement of their elastic tissue, manifest movements, prior to their rupture, which have long excited the surprise and admiration of the physiological observer.
A large vitellus, among the numerous nucle ated cells of which it is often difficult to recog nize a single one as the centre of development, or as the germinal vesicle, characterizes the ovum of most Mollusks. In most species also the early formation of vibratile cilia on the surface of the germinal membrane, and the rotation of the embryo upon its axis produced by their action, are striking though not peculiar phenomena. The adherence to a uniform type in the earlier pe riods of growth is singularly manifested in the young of Tritonia, Doris, Aplysia, and other naked Mollusks, which are protected for a cer tain period by an external spiral univalve shell.
The classification of the Molluscous animals has exercised the judgment and discrimination of some of the ablest Zoologists, and is a sub ject too expanded for the limits assigned to the present article. The principles of a natural distribution into the larger groups according to general organization seem to be adhered to in the following system.
Taking the nervous system as a guide to the divisions of highest value and extent, the Molluscs separate themselves, as already shown, into ACEPIIALA and ENCEPHALA.
The Acephala may be divided according to the nature of their external covering into Ty cATA, where this is continuous, flexible, and elastic ; and into CONCHIFERA, where it is tes taceous and divided into two or more valves. The Conchifera may be subdivided according to the modifications of the respiratory organs into Palliobranchians and Lamellibranehians. Respiratory characters likewise mainly distin guish the sessile Tunicaries or Ascidians, and the floating Tunicaries, or Salpaceans.
The Encephalous Mollusks are classified ac cording to their organs of locomotion, as PTE.