THE ANATOMY OF THE LARVA 51 



than the diameter of the tube, being placed in the centre 

 of a small circular plate supported by chitinous rays. The 

 mechanism by which these openings are protected from 

 the entry of water is rather complex and consists of a sort 

 of valve, formed by five flap-like lobes at the end of the 

 process. These flaps consist of two pairs and a small 

 unpaired lobe, the larger of the two pairs being placed 

 opposite the small unpaired one. Between the two air 

 tubes and around them are a number of muscular strands, 

 which originate in the eighth segment and extend through 

 the process to be inserted into the bases of the flaps, so 

 that, when they contract, the stigmata are drawn down 

 somewhat within the process, and the valves close over 

 them. 



Vascular System. — This consists of a delicate, long, wide 

 tube, which extends along the dorsal aspect of the body, 

 from the hinder part of the head to the very extremity of 

 the abdomen. Its structure is so delicate that, except in the 

 living larva, where its shape and position can be followed by 

 its movements, it is extremely difficult to make out. It 

 consists essentially of a long, thin-walled tube, with valvular 

 constrictions opposite the incisurae, communicating freely 

 with the perivisceral spaces by means of cribriform aper- 

 tures, which, however, are anything but easy to demonstrate. 

 There is no system of peripheral vessels at all comparable 

 with those of the higher animals, the function of the organ 

 being to keep in motion the perivisceral fluid, which is 

 thus kept flowing over the tracheae, which lie free in the 

 perivisceral spaces between the various organs and tissue 

 elements. It acts, in fact, rather as a churn than as a 

 pump. In front of the thorax it extends as a narrower 

 vessel into the head, and this portion is sometimes spoken 

 of as the " aorta," but I look upon this term as a misnomer, 

 as, save in diameter, the extension difl^ers in no way from 

 the rest of the dorsal vessel, is quite unprovided with 

 branches, and appears at its anterior end to lose itself in 

 the general interstitial lymph spaces of the head. The 

 circulating fluid is colourless and contains but few cellular 

 elements, those that are present being of the nature of 



