CRUSTACEA 311 



no doubt coelomoducts, homologous with those of the AnneHda, 

 their ectodermal portions probably are not the homologues of 

 nephridia but represent ectodermal glands such as are common in 

 the Crustacea. Various other glands, mostly of doubtful morpho- 

 logical significance, which occur in different crustaceans have been 

 shown, or are suspected, 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 

 replaced 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 in- 

 creased by branching or folding (Figs. 21 1 F ; 272 ; 274, 1,2). In the 

 Decapoda incorporation of the precoxa with the flank of the body 

 has brought it about that some of the gills (proepipodites, p. 299), 

 stand in that position and not upon the actual limbs (Fig. 221). 

 Such gills are known as "pleurobranchiae". In the Isopoda re- 

 spiration 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 wood- 

 lice, which are terrestrial members of the Isopoda, are remarkable 

 in having adopted the principle of respiration employed by normally 

 terrestrial arthropods, for the integument of their abdominal 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. 225). 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 



