TIII-: AKTIIKOPODA. 221 



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

 glands. Tin- anus, \\ Inch is very rarely absent, is situated in 

 the hindermost somite. 



In lik< miniiirr, the blood-vascular system varies from a 

 mi -re perivisceral cavity without any heart ( Ostracoda, Cirri- 

 ptdiaj 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 lacuna?. 

 The blood-corpuscles are colorless, nucleated cells. 



Special respiratory organs may be absent, or they may 

 take one <>f the following forms: 



1. I>ruchioB. Externally projecting processes of tin- 

 body or limbs, supplied with venous blood, which \A thus 

 brought into contact with the air dis>olv'd in \\ater. 



2. Tracheae. Tubes which traverse the body and gen- 

 erally open upon its exterior by apertures termed stigmata, 

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

 generally. Saccular reservoirs of air are often formed by 

 dilatations of these tubes. 



The so-called Trackeo-branchicB 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 whirh traverse 

 the body (Ephemeridoe, Perlaridce). They are in no sense 

 Inane-Ilia 1 , but simply take the place of stigmata. The ex- 

 change 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-branchiae, instead of directly, through the stigmata, 

 as in other cases. 



In the aquatic larvae of many Dragon-flies (LibeUulidai), 

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

 the lining membrane of the rectum, which contain abundant 

 trachea.'. Water is drawn into, and expelled from, the ca\ it v 

 of the rectum by rhythmical contractions of it> \\alls, so as 

 t -i cure the exchange of gaseous constituents between the 

 air which it contains and that which fills the tiacheie. 



3. rt//mntt,rr>/ ,wx. These are met with nnly in some 

 Ara<'/nn't?<i. They are involutions of the intelligent, tin- 

 walls of which are folded in such a manner a- se a 

 lar^e 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- 



