MUSCULATURE AND CONNECTIVE TISSUES. 81 



According to our present anatomical and ontogenetical knowledge, 

 the communication between the pericardia] cavity and the blood- 

 vascular system which was formerly assumed, does not exist. The 

 idea of an admixture of water with the blood which was also held 

 must be regarded as exploded, quite apart from the fact that the 

 transmission of water from outside through the organ of Bojanus 

 into the pericardium seems from recent researches to be highly im- 

 probable (Rankin). The structure of the organ itself as well as the 

 direction of the cilia within it are unfavourable to such a process. 

 Indeed the whole idea of the reception of water into the body of the 

 Lamellibranch from without, which has often been adopted as an 

 explanation of the swelling of the foot, must be regarded as refuted. 

 The pores which were supposed to conduct water from without into 

 the foot could not be demonstrated ontogenetically (Ziegler). The 

 swelling of the foot, as is evident from the statements of a number of 

 authors (Carriere, Fleischmann, Schiemenz, Rankin, etc.), is 

 rather due to the fact that the greater part of the blood is driven 

 into this organ. This is brought about through the blood being- 

 retained in the foot, the valve at the entrance to the sinus venosus 

 being closed and the blood which was emerging from the foot being 

 thus retained within it. Besides this, the quantity of blood already in 

 the foot is increased through the flow of fresh blood from the anterior 

 aorta. When the foot is extended, the sphincter at the point where 

 the posterior aorta emerges from the heart contracts, so that the 

 greater part of the blood is obliged to flow through the anterior aorta 

 into the foot. During this process, a certain amount of blood still 

 circulates in the heart, so as to prevent an arrest of the whole circula- 

 tion. When the valve in the sinus venosus opens, the blood flows 

 out of the foot, and as the latter ceases to extend, the sphincter of 

 the posterior aorta opens again, until, when the animal moves on 

 again the same process is repeated. 



G. Musculature and Connective Tissue. 



The only organs as yet referred to as differentiations of the mesoderm 

 have been the coelom, the kidney and the blood-vascular system, but 

 there are other structures mesodermal in origin, which, indeed, up to 

 the present have received little attention from zoologists; these are the 

 musculature and the connective tissue, and, further, the genital organs. 

 which will be dealt with immediately. The muscle-cells are formed 

 by the detachment of single cells from the mesoderm-mass, the distri- 



G 



