THE ARTHROPODA. 221 



in form and structure, and in the number and nature of its 

 glands. The anus, which is very rarely absent, is situated in 

 the hindermost somite. 



In like manner, the blood-vascular system varies from a 

 mere perivisceral cavity without any heart ( Ostracoda, Cirri- 

 pedicC) up to a complete, usually many-chambered heart with 

 well-developed arterial vessels. The venous channels, how- 

 ever, always have the nature of more or less definite lacunae. 

 The blood-corpuscles are colorless, nucleated cells. 



Special respiratory organs may be absent, or they may 

 take one of the following forms : 



1. Branc1ii<x, Externally projecting processes of the 

 body or limbs, supplied with venous blood, which is thus 

 brought into contact wdth the air dissolved in water. 



2. Tracheae. Tubes w^hich traverse the body and gen- 

 erally open upon its exterior by apertures termed stigmata, u«x,^ 

 and thus bring air into contact with the blood and the tissues 

 generally. Saccular reservoirs of air are often foim.ed by 

 dilatations of these tubes. 



The so-called Tracheo-hranchim of some aquatic Insect 

 larvae are usually laterally projecting processes of more or 

 fewer of the thoracic or abdominal somites, containing abun- 

 dant tracheae, which communicate with those which traverse 

 the body [^Ephemeridce, Perlaridaf). They are in no sense 

 branchiae, but simply take the place cf stigmata. The ex- 

 changee of constituents between the air contained in the 

 tracheae of these animals and that of the surrounding medium 

 is effected indirectly, by diffusion through the walls of the 

 tracheo-branchia?, instead of directly, through the stigmata, 

 as in other cases. 



In the aquatic larvre of many Dragon-flies (Zibelhdidc^), 

 the function of the tracheo-branchiae is performed by folds of 

 the lining membrane of the rectum, which contain abundant 

 tracheae. Water is drawn into, and expelled from, the cavity 

 of the rectum by rhythmical contractions of its walls, so as 

 to secure the exchanjre of g-aseous constituents between the 

 air which it contains and that v.'hich fills the tracheae. 



3. Pulmoiiary sacs. These are met with only in some 

 Arachnida. They are involutions of the integument, the 

 walls of w^hich are folded in such a manner as to expose a 

 large surface to the air, which is alternately taken into, and 

 expelled from, their apertures. The blood is brought to these 

 sacs by venous channels. 



The exact mode by which the separation of the nitro- 



