cardiac chamber and cesophageal valve, the absence of any distinct 

 enlargement at the anterior end of the small intestine, the large thin- 

 walled colon, and the very short malpighian tubules. 



RESPIRATION AND CIRCULATION. 



Tracheae. Before describing the tracheal system of C. pusio we must glance 



at the general arrangement found in most of these larvae. Nearly all 

 the different species that I have examined (the C. Pusio group excepted) 

 possess fully developed longitudinal trunks beginning in or near the 

 head, and passing down one on each side of the body to the last 

 segment. Lateral branches pass off in each segment, ramify on the 

 body-wall, or penetrate into the brain or other internal organ. The 

 trunks vary in thickness, in the amount of pigmented epithelial cells 

 surrounding them, and in the number of branches given off, but the 

 general plan is always the same. 



Now in C. Pusio and the rest of this group the main trunks are 

 only represented in the thorax and in the last three abdominal segments. 



The thoracic tracheae consist of a pair of main trunks in the first 

 segment ; these pass forwards into the head and end in fine ramifications, 

 and push backwards into segment 2, where they end in a similar 

 manner. About the middle of segment i a stout branch is given off, 

 which soon ends abruptly in a mass of radiating twigs spread over the 

 the dorsal surface. When we remember that this segment is the only 

 part of the body that is exposed to the full force of the water (the rest 

 of the body being protected by the larval tube) we see a reason for this 

 distribution. It is probable then that this cutaneous network of tracheal 

 branches is functional and acts as a tracheal gill, absorbing oxygen 

 from the water and conveying it to the internal organs. The main 

 internal branch passes to the brain, which is exceptionally large, and 

 may require a special supply of oxygen. If this is the case we have here 

 a case of a degenerating organ once more regenerating itself. The 

 posterior trunks begin in the middle of segment 10, and pass down 

 to the anal gills where they end in fine twig-like ramifications. A few 



