74 T. C. WHITE ON THE HISTOLOGICAL DEVELOPMENT 



vessel commences in the tenth segment, and terminates in what we 

 might call the neck of the larva ; it is composed of two coats of 

 great tenuity, the internal lining having delicate longitudinal muscles, 

 and the external coat composed of circular fibres, but it is so dia- 

 phanous that it is only by very careful manipulation of the light 

 that these details can be made out. It seems to float freely in the 

 body cavity, being held in suspension by very fine muscles which, 

 like guy- ropes, attach its walls, allow of free movement, and become 

 merged in the sheath of the sympathetic ganglion. In the ninth 

 segment these muscles form quite a fan-shaped leash of threads. 

 At its commencement in the tenth segment, it has an opening 

 guarded by a large valve, and at its termination in the thorax is 

 another opening, round which a band of circular muscular fibres 

 acting like a sphincter are placed. There are about eight valves in 

 the abdominal segments, but none in the thorax, but the coats of 

 the vessels in this portion are more muscular, and it seems to have 

 the power of contracting longitudinally to such a degree as to close 

 itself almost entirely ; it does not seem to act so rhythmically as 

 the abdominal portion. There are several valves in the abdominal 

 portion, but on account of the free movement of the vessel, the focal 

 plane is being constantly shifted, rendering it a matter of consider- 

 able difficulty in using a high power to see the details of their 

 mechanism, but it appears to me that their shape may be compared 

 to a truncated cone, opening inwards into the vessel, and closed 

 by the sudden collapse of its two sides. That openings through 

 these valves do exist, I have proved by seeing blood corpuscles 

 drawn through them in the act of expansion. The blood, which is 

 very sparingly supplied with corpuscles, can be detected slowly 

 moving through hollow spaces between the viscera, and it conse- 

 quently bathes the external surfaces of them till being drawn by the 

 expansions into the dorsal vessel, it is impelled towards the head, 

 when it again slowly descends through these cavities, and the pro- 

 cess is repeated ; thus you see the circulation is of the simplest 

 description. 



The alimentary system, commencing as we have seen in the first 

 segment by the mouth and dilated pharynx, is continued about as far 

 as the junction of the thorax and first abdominal segment, where the 

 pharynx terminates in a gizzard ; this gizzard is formed externally 

 by several rings of powerful muscles, while the interior is furnished 

 with a membrane having a close array of chitinous ribs and two 



