58 COMPARATIVE ANATOMY. 



A blue injection has been thrown into the gills, which are 

 four in number on each side. Each gill carries on its outer 

 or convex edge a double row of short, flattened, vascular fila- 

 ments, and on its inner or concave side a row of short bony 

 tubercles. In the angle between the converging gills are to be 

 seen the central parts of the circulatory system, consisting of a 

 triangular ventricle, leading into the pulmonary aorta anteriorly, 

 an auricle which slightly overlaps the ventricle, and a 'sinus 

 venosus,' into which the great veins pour their blood. The 

 heart is separated from the subjacent liver by a transverse 

 peritoneal partition. The latter viscus is of large size, rounded 

 in front and above, or divided inferiorly by a deep fissure into a 

 right and left lobe. Depending as it were from beneath the 

 inferior edge of the liver is the caecal stomach, with the intes- 

 tine, to which are appended three pyloric caeca passing out of it 

 at right angles. After describing an S-shaped convolution, the 

 intestine expands into a dilated, thin- walled, and transparent 

 rectum. Behind the rectum is the single large, pear-shaped 

 ovary, and still more posteriorly the small oval bladder. The 

 spleen lies in a loop of intestine, and a portion of it, stained 

 with the blue injection, is to be seen between the lower edge of 

 the stomach and the ovary. The large semi-transparent air-sac, 

 closely applied to the hinder wall of the abdomen, has been 

 left intact in the greater part of its extent ; but the posterior 

 third has been removed, in order to expose a portion of the 

 kidney, which lies between it and the vertebral column 

 throughout the whole extent of the abdomen. A slip of blue 

 paper has been placed beneath one of the ureters. The anus is 

 situated anteriorly to the openings of the generative and urinary 

 ducts, and not posteriorly, as in the higher Yertebrata. 



In the brain the cerebral hemispheres are small, and their 

 surfaces are slightly corrugated. From their anterior extremi- 

 ties the olfactory nerves are given off. The optic lobes, which 

 succeed them, are the largest divisions of the brain : and by their 

 opposition to the hemispheres in front, the optic thalami are 

 hidden from view. The next division is the small, smooth cere- 

 bellum, and the fourth ventricle which it bounds anteriorly is 

 widely open. (Rolleston, p. 40.) 



