470 INVERTEBBATE MORPHOLOOT. 



the appendages. A marked difference between the Tracheata 

 and the Arachnida is the universal absence of an endo- 

 sternite, a structure of considerable phylogenetic significance 

 in the latter group. A heart is invariably present, lying 

 above the intestine, and situated in a pericardial siaus incom- 

 pletely partitioned off. In the majority of forms the parti- 

 tion is composed of a varying number of triangular muscles, 

 the alar muscles, which are attached by their bases to the 

 walls of the heart, and by their apices to the body-wall. 

 While at rest they are somewhat vaulted, the convexity being 

 dorsal, and on contraction flatten down, thus enlarging the 

 sinus and causing a flow of blood into it. The heart (Fig. 

 227, Ji) is elongated and imperfectly divided into a series of 

 chambers, separated by pairs of valves which allow the blood 

 to flow from behind forwards but not in the reverse direction, 

 the heart being closed behind. Ostia are present in the 

 lateral walls to allow of the entrance of blood into the heart- 

 chambers, whence it is propelled through very short arteries 

 which open widely into the lacunar spaces of the ccelom. In 

 many forms a ventral sinus surrounds the ventral ganglionic 

 nerve-chain, the blood flowing in it from before backwards, 

 but with this exception definite vessels are wanting. This is 

 compensated for by the rich branching of the tracheae, which, 

 as stated, serve as respiratory organs and convey air to all 

 parts of the body ; the air is in fact brought directly to the 

 tissues, instead of being carried to them by the blood from 

 limited portions of the surface of the body. The blood is 

 usually colorless, but in some cases is of a bright yellow or 

 green color, owing to pigment contained in the plasma, and it 

 contains in all cases colorless amoeboid corpuscles. 



The tracheae (Fig. 215, tr) communicate with the exterior 

 along the sides of the body by a varying number of pairs of 

 stigmata {st), and may either consist of bunches of unbranched 

 tubes connected with each stigma, or of a number of richly- 

 branching tubes, each one arising from a separate stigma and 

 anastomosing in some cases through some of its branches 

 with the tubes from other stigmata. Each stigma is usually 

 provided with an apparatus by which it may be closed, and 

 in the Insects the air is expired from the tracheae by the con- 



