25 



branches are given off laterally ; these chiefly end in the walls of 

 the colon. One would naturally suppose that this system was the chief 

 respiratory one, but as the posterior end of the body is very rarely 

 exposed outside the case (chiefly to evacuate) this can hardly be so. 



The heart lies in segments 10 and n. It is a long hour-glass Heart and Aorta. 

 shaped organ, almost divided into two chambers by the central narrowing 

 (fig. io/ and i). There are posterior and anterior pairs of lateral inlets 

 or ostia, and alary muscles pass off laterally from each chamber. From 

 the anterior end, guarded by a pair of valves, arises the aorta ; it is 

 a transparent tube, without valves, non-contractile, and passes forward 

 along the dorsal wall of the alimentary canal towards the head. The 

 dorsal vessel thus resembles the type except for the median constriction 

 of the heart. A second form of dorsal vessel is also known in which 

 the aorta is contractile and valved, but it does not seem very common, 

 only two of my species (No. n and No. 12) possessing this form of 

 blood-vessel. 



The blood in the young larva is quite transparent, in the older 

 ones it becomes greenish or even slightly reddish. In larva No. 5, 

 a very closely related 'species, though much larger, the blood goes 

 through an additional phase. When the larva is about the size of 

 C. pitsio the blood is greenish, but by the time of full growth (2 cm. 

 long) it has become a deep blood-red. 



THE FAT SACS. 



In segment io are found a pair of oval bodies which have not 

 as far as I am aware been hitherto described. They are very variable 

 in size, and not always easy to see on account of their transparency, 

 and the fact that they are often hidden by the fatty tissue generally 

 very plentiful in this region of the body. Having found them in one 

 larva, I examined many other species for the same organ, and found 

 that they were by no means rare, and I have also noticed the same 

 organ in a small undescribed species of Tanypus. All the larvae of the 

 "Pusio" group possess them. In No. 18 they are rather small but 



