BY R. J. TILLYARD. 63 



found that this is only true for the lateral gills. In the median 

 gill, the circulation is reversed, the blood passing in dorsally, 

 and passing out again ventrally. As will be seen in Text-fig. 4, 

 the blood that enters the median gill (the line of arrows marked 

 S in the figure) is part of the dorsal backward blood-stream. On 

 reaching the base of the gill, a single pulsation of the heart 

 causes it to shoot sharply along the narrow dorsal blood-canal 

 for some considerable distance. It then travels by a series of 

 shorter jerks (each corresponding with one heart-beat) along the 

 dorsal canal, and round the distal confluence {cf) into the ventral 

 canal. Along the ventral canal it passes forwards again by short 

 jerks, until at last it passes out of the gill in the same sharply 

 shooting manner as that by which it entered. The distance 

 covered by a given corpuscle in the jerks of entry and exit 

 appeared to me to be between two and three times as long as 

 the distance covered in a single ordinary jerk. I think the 

 increase must be wholly due to increased pressure forcing the 

 ])lood along at the breaking-joint, where the canals become sud- 

 denly narrowed — just in the same way that a river flows fastest 

 in the narrowest part of its bed. 



Thus we see that, in the 'inedian gill, the dorsal hlood-canal is 

 afferent and primary, the ventral hload-ca^ial efferent and 

 secondary. 



Turning now to the lateral gills, the course of the blood in the 

 left lateral gill is marked by the line of arrows marked 7 in 

 Text-fig. 4. This is part of the main ventral backward stream 

 of the blood along the left side of the abdominal haemocoele. A 

 similar stream along the right side supplies the right lateral gill. 

 Each of these streams enters the lateral gill on its own side by 

 the ventral canal, in the same sudden shooting manner as that 

 already noticed for the median gill. Passing, by a series of 

 jerks, distally to the confluence {cf), the blood returns along the 

 dorsal canal. On passing out from the gill, the blood passes 

 obliquely upwards and forwards towards the heart, and meets 

 with one portion of the blood-stream emerging ventrally from the 

 median gill, since this latter stream is constrained, on approach- 

 ing the heart, to divide into two in order to avoid the blind end 



