This theory of origin is strengthened by closer observation of the phenomena of a single group. Taking the Septibranchiata as an instance, we find that in Lyonsiella the branchiae unite with the mantle in such a way as to form two large pallial chambers, the structure of the branchiae being preserved, and their lamellae covering the partition. A further stage is observed in Poromya. There, a similar partition exists, but it has become muscular, preserving, however, on each side two groups of branchial lamellae, separated one from the other by a series of slits, which form a communication between the two pallial chambers. A further stage still is seen in Silenia. There the same muscular partition exists, but the branchial lamellae on either side have disappeared, the slits between the two chambers, which occur in Poromya, still persisting, but separated into three groups. Cuspidaria represents the last stage in the development. In the ventral chamber there appears nothing at all corresponding to a branchia; the surface of the partition appears perfectly uniform, but on careful examination three little separate orifices, remains of the three groups of orifices in Silenia, are observed.[276]
Relation between Branchiae and Heart.—The object of the branchiae being, as has been already stated, to aerate the blood on its way to the heart, we find that the heart and the branchiae stand in very important structural relations to one another. When the branchiae are in pairs, we find that the auricles of the heart are also paired, the auricle on the right and left sides being supplied by the right and left branchiae respectively. This is the case with the Dibranchiate Cephalopods (Argonauta, Octopus, Loligo, etc.), the Zygobranchiate Prosobranchs (Fissurella, Haliotis), and all Pelecypoda. In the Amphineura (Chiton, etc.) there are two auricles corresponding to the two sets of multiple branchiae. In the case of the Tetrabranchiate Cephalopods (Nautilus) there are four auricles corresponding to each of the four branchiae. Compare Fig. [79], A, B, C, D, E.
On the other hand, when the branchia is single, or when both branchiae are on the same side, and one is aborted and functionless, the auricle is single too, and on the same side as the branchia. This is the case with the Tectibranchiate Opisthobranchs (Philine, Scaphander, etc.), all the Pectinibranchiate Prosobranchs (Rachiglossa, Taenioglossa, and Ptenoglossa), and the other Azygobranchiate Prosobranchs (Trochidae, Neritidae, etc.). In the last case the right auricle exists, as well as the left, but is simply a closed sac, the coalescing of the two gills on the left side having thrown all the work upon the left auricle. Compare Fig. [79], F, G, H.
Fig. 79.—Diagram illustrating the relations between branchiae, heart, and aorta in the Mollusca: A, In Chiton; B, Pelecypoda; C, Dibranchiate Cephalopoda; D, Tetrabranchiate Cephalopoda; E, Prosobranchiata Zygobranchiata; F, Prosobranchiata Azygobranchiata; G, Prosobranchiata Monotocardia; H, Opisthobranchiata Tectibranchiata: 1, Ventricle; 2, Auricle; 3, Aorta; 3a, Cephalic aorta; 3b, Visceral aorta; 3c, Posterior aorta. (From A. Lang.)
Circulatory System
All Mollusca, without exception, possess a circulatory system of more or less complexity. The centre of the system is the heart, which receives the aerated blood from the breathing organs, and propels it to every part of the body. In the Scaphopoda alone there appears to be no distinct heart.
The heart may consist simply of a single auricle and ventricle, and an aorta opening out of the ventricle. From the aorta the blood is conveyed to the various parts of the body by arteries. Veins convey the blood back to the breathing organs, after passing over which it returns by the branchial or pulmonary vein to the heart, thus completing the circuit.
As regards position, the heart is situated within the pericardium, a separate chamber which in the Pelecypoda, Cephalopoda, and the bilaterally symmetrical Gasteropoda lies on the median line, while in the asymmetrical Gasteropoda it is on one or other of the sides of the body, usually the right. The veins connected with the branchiae, and consequently the auricle into which they open, are situated behind the ventricle in the Opisthobranchiata (whence their name), while in the Prosobranchiata they are situated in front of the ventricle.
The number of auricles corresponds to the number of branchiae. Thus there is only one auricle in the great majority of Prosobranchiata (which are accordingly classified as Monotocardia), and also in the Opisthobranchiata, while the Pulmonata have a single auricle corresponding to the pulmonary chamber. There are two auricles in the Amphineura, in a small group of Gasteropoda, hence known as Diotocardia, in all Pelecypoda, and in the Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda alone there are four auricles corresponding to the four branchiae.