Introduction to Animal AlorpJiology. 393 



true pericardium for its collection. In the limbs defi- 

 nite blood channels are traceable, and in the wings 

 true walled vessels and capillaries have been de- 

 scribed. 



The aorta is an anterior chamber of the heart, without 

 venous ostia, or ligamenta alaria ( PFmi-^waw). The heart 

 muscle is not in discriminable strata. It acts more rapidly 

 in the adult than in the immature stages. In Sigara coleop- 

 trata there is an accessory contractile space in the tibia. 

 The heart is a simple tube in some Dipteran and Hymenop- 

 teran larvae. 



Respiration is by various kinds of tracheae. Some 

 aquatic insects have a closed system, with no stig- 

 mata — a breathing organ differing from gills, in that 

 the blood is aerated by oxygen separated from the 

 water, and conveyed in tubes to the blood. The tubu- 

 lar system in the gill covers of Oniscus is closely re- 

 lated hereto. In Phryganea and some Dipteran larvae 

 there is a sub-dermal free branching of tubular or 

 lacunary air channels, the dermis separating the air 

 from the surrounding medium. A second breathing 

 organ consists of leaf-like surface processes or areas 

 containing these closed tracheae (tracheal gills before 

 referred to) ; these form a lateral row along the body 

 of an Ephemeron larva and in other Pseudoneuro- 

 ptera and Neuroptera, and persist in the Orthopteron, 

 Pteronarcys regalis, which has thirteen pair. A third 

 form of closed tracheae is found in Dragon-flies, &c., 

 where the internally placed tracheae ramify on ridges 

 within the rectum. Wings may be permanent, homo- 

 logues of these external gills. Open tracheae are 

 most common, and they vary in number, and in the 



