348 THE INVERTEBRATA 



pected, to have an excretory function. Thus, in Nebalia, eight pairs 

 of ectodermal glands at the bases of the thoracic limbs are excretory, 

 while in ostracods a pair of rather complex glands, also of ectodermal 

 origin, which lie between the folds of the shell in the antennal region, 

 may have a similar function. Excretion appears also sometimes to 

 be performed by coeca of the mid gut — as by some of those of the 

 barnacles and by the posterior pair of amphipods — or by cells of 

 the epithelium of the mid gut itself. 



Respiration in many of the smaller crustaceans, notably in the Cope- 

 poda, takes place through the general surface of the body. In forms 

 with stouter cuticle or more bulky bodies this is supplemented or re- 

 placed by the use of special organs upon which the cuticle remains 

 thin. The most important of such organs are the lining of the cara- 

 pace, if that structure be present, and certain epipodites which are 

 known as gills and in many of the Malacostraca have their surface 

 increased by branching or folding (Figs, 222 F; 285; 287, i, 2). In 

 the Decapoda incorporation of the precoxa with the flank of the body 

 has brought it about that some of the gills (proepipodites, p. 337), 

 stand in that position and not upon the actual limbs (Fig. 232). Such 

 gills are known as "pleurobranchiae". In the Isopoda respiration is 

 effected by the broad rami of the abdominal limbs. Renewal of the 

 water upon the respiratory surfaces may be brought about by the 

 movements of the limbs upon which they are located, but often certain 

 appendages bear special lobes adapted to set up a current under the 

 carapace and thus to flush the chamber in which the gills and the 

 carapace lining are situated. 



Some land crustaceans have no special adaptations for respiration 

 in air. In others the gill chamber is adapted, by the presence of 

 vascular tufts of the lining of the carapace, for use as a lung. The 

 woodlice, which are terrestrial members of the Isopoda, are remark- 

 able in approaching in their respiration the principle employed by 

 normally terrestrial arthropods, for the integument of their ab- 

 dominal limbs is invaginated to form branching tubes which resemble 

 tracheae. 



The vascular system is seen in its most primitive condition in the 

 Branchiopoda Anostraca (Chirocephalus, Fig. 236). Here the heart (h.) 

 runs the whole length of the trunk, situated above the gut in a blood 

 sinus known as the pericardium, with which it communicates by a 

 pair of ostia in each somite except the last. In front it is continued 

 into the only artery, a short aorta, from which the blood flows direct 

 into the sinuses of the head and thence through those of the trunk to 

 the pericardium, eddies from a main ventral sinus supplying the 

 limbs. In all other Crustacea, except the Stomatopoda, the heart, if it 

 be present, is in some degree shortened, and in the Malacostraca (Fig. 



