44 ANATOMY AND PHYSIOLOGY OF RALPA AND PYROSOMA 



band " (x) runs upwards, until it meets with its fellow of the opposite 

 side, the two thus encircling the anterior aperture of the respiratory 

 cavity. 



1 8. The dorsal wall of the respiratory cavity is marked by two 

 longitudinal folds, running from before backwards to the mouth. 

 These are the dorsal folds of Savigny and others ; but there is an 

 organ to which the name of " Endostyle " may be given (c), very 

 distinct from these, and yet which has been invariably confounded 

 with them, consisting of a long tubular filament, with very thick 

 strongly refracting walls, Plate XV. [Plate 5] fig. 4 ^- This body lies 

 in the dorsal sinus ; its anterior extremity is slightly curved down- 

 wards, somewhat pointed, and looks stronger and more developed 

 than the posterior extremity, which is paler, more delicate and 

 truncated. By its ventral surface this " endostyle " is attached to 

 a ridge of the inner tunic, which rises up into the dorsal sinus. 



19. It has been stated that the circulatory system consists, not of 

 vessels with distinct parietes, but of more or less irregular sinuses. 

 However irregular in form, the position of several of these is very 

 constant. There is a dorsal sinus running along the dorsal surface and 

 enclosing the internal shell ; there is a ventral sinus opposite to this and 

 containing the ganglion ; there are lateral sinuses connecting these. 

 Then there is the sinus in which the intestine and generative organs 

 lie, the peri-intestinal sinus, and, finally, the sinus which, connecting 

 the dorsal and ventral system of sinuses, traverses the gill and con- 

 stitutes the branchial sinus. 



These sinuses all communicate together round the oesophagus, and 

 above and in front of this, the heart {g) is developed. The heart 

 lying obliquely at the posterior extremity of the dorsal sinus, is not 

 tubular, as it has been described ; it forms not more than three-fifths 

 of a tube ; nor is it correct to say that it lies in a pericardium. Its 

 true nature will be best conceived by supposing the inner surface of a 

 .sinus to have become developed for about three-fifths of its circumfer- 

 ence into a free muscular membrane, Plate XV. [Plate 5] fig. 9. 



This membrane is exceedingly delicate, and is composed of a single 

 layer of flat striated muscular fibrils. 



20. The direction of the circulation depends entirely upon the order 

 of contraction of the muscular fibrils of the heart. If they contract 

 successively from behind forwards, the blood is forced in that direc- 

 tion ; after a certain number of such contractions, they all become 

 simultaneously, as it were, paralysed for a short period, and then they 

 begin to contract again, but in the inverse order, and of course with 

 an opposite effect upon the direction of the circulation. 



