130 ME. J. M. BKOWN ON THE ANATOMY OF 



Reviewing the system in the larva of T. maxima, we notice : — 



1. An extensive system of fine air-tubes taking origin from the stigmatic chambers 

 and radiating to the body- wall. Amongst these tubules the blood circulates. It may be 

 suggested that this is an adaptation for bringing the blood into relation with the air near 

 to the spiracles themselves. 



2. The presence of blood-gills for respiration when submerged. 



In the terrestrial form T. oleracea. — (1) The spiracle structure and disposition of 

 air-tubes agree with the above, the stigmatic chambers being, however, shorter. The 

 same necessity for the arrangement exists here as in the above. 



(2) Blood-gills are not developed. 



In an undetermined aquatic form, which, however, appears like T. eluta (see 

 PL 26. figs. 21 & 22). — 1. A pair of spiracles occur on the posterior face of the 

 hind segment, as in T. maxima. They also open into a stigmatic chamber leading into 

 the longitudinal air-tubes. There is, however, a difference in construction. The cover 

 of the spiracle consists of a solid conical plug of chitin which fits into the spiracle and 

 projects downwards into the chamber like a stopper, but leaves a comparatively narrow 

 border on all sides. This is supported from the rim of the spiracle by a single narrow 

 circlet of chitinous props, consisting of very short radial bars separated from each other 

 by narrow slits (like the outer radial bars in T. maxima). These are supported directly 

 by upright pillars without the intervention of Y-pieces (PI. 26. fig. 22). Thus we have 

 a very narrow border to the spiracie, through which air can enter. Further, the filtering- 

 hairs, arising from the chitinous lining to the chamber, traverse the distance between 

 the wall and stopper and unite the one with the other. Side branch threads occur 

 forming an efficient filter which would prevent the entrance of water. (A very similar 

 structure is described in the larva of Dicranota bimaculata by Miall.) No bunches 

 of air-tubes pass from this chamber to the body-wall. This part of the system appears 

 therefore simplified. 2. The ventral blood-gills are, however, much more strongly 

 developed, and, as the creature is usually submerged, will play a more important part in 

 respiration. 



Circulation. 



The dorsal vessel, or "heart" (PL 27. fig. 19, h.), occupies the usual position in the 

 mid-dorsal line of the body. The chambered portion extends from the 4th segment to 

 the 11th (hindmost), and includes eight chambers. Anteriorly it is continued as a narrow 

 uniform tube (" aorta ") as far as the cerebral ganglia. The dorsal vessel lies imme- 

 diately upon, and is organically connected with, the muscular pericardial diaphragm. 

 In certain parts the wall of the heart and the diaphragm cannot be distinguished from 

 each other. The heart is further supported by branches of the tracheal system. The 

 diaphragm (PL 27. fig. 19, d.) extends from the posterior segment as far forwards as the 

 middle of the fourth. It is supported from the body-wall by the alary muscles, which 

 are attached not only at the junctions between segments but also at the middle of each 

 segment. In the posterior segment the diaphragm turns ventralwards, and serves to 

 cut off the posterior region as a special sinus. 



