Body cavity and vascular system. The body cavity in Gasteropods and Pteropods originates either by a definite splitting of the mesoblast, or by the appearance of intercellular spaces. It becomes divided into numerous sinuses which freely communicate with the vascular system. Very different accounts have been given by different investigators of the development of the heart in the Gasteropoda and Pteropoda. It would seem however in most cases to arise as a solid mass of mesoblast cells at the hind end of the pallial cavity, which subsequently becomes hollowed out and divided into an auricle and ventricle. Bobretzky's careful observations have fully established this mode of development for Nassa. In Pteropods the heart is formed (Fol) close to the anus, but slightly dorsal to it (fig. 108, _h_). The pericardium is formed from the mesoblast at a considerably later period than the heart. A very different account of the formation of the heart is given by Bütschli for Paludina. He states that there appears an immense contractile sack on the left side of the body. This becomes subsequently reduced in size, and in the middle of it appears the heart, probably from a fold of its wall. The original sack would appear to give rise to the pericardium. In connection with the vascular system mention may be made of certain contractile sinuses frequently found in the larvæ of Gasteropoda and Pteropoda. One of these is placed at the base of the foot, and the other on the dorsal surface within the mantle cavity immediately below the velum[114]. The completeness of the differentiation of these sinuses varies considerably; in some forms they are true sacks with definite walls, in other cases mere spaces traversed by muscular strands. They are found in the majority of marine Gasteropods, Heteropods and Pteropods. In Limax a large posteriorly placed pedal sinus is well developed, and there is also a sinus in the visceral sack. The rhythmical contraction of the yolk-sack of Cephalopods appears to be a phenomenon of the same nature as the contraction of the foot sinus of Limax. [114] Rabl holds that there is no contractile dorsal sinus, but that the appearance of contraction there is due to the contractions of the foot. In Calyptræa (Salensky) there is an enormous provisional cephalic dilatation within the velum which does not appear to be contractile. Similar though less marked cephalic vesicles are found in Fusus, Buccinum and most marine Gasteropods. In Cephalopods the vascular system is formed by a series of independent (?) spaces originating in the mesoblast, the cells around which give rise to the walls of the vessels. The branchial hearts are formed at about the time at which the shell-gland becomes closed. The aortic heart (fig. 127, _c_) is formed of two independent halves which subsequently coalesce (Bobretzky). The true body cavity arises as a space in the mesoblast subsequently to the formation of the main vascular trunks.