336 Introduction to Aminal Morphology. 



ventral vein, passing in the sternal canal to a central sinus, 

 from which branchial arteries convey it to the gills, and from 

 thence branchial veins pour it into the pericardial sinus. 



The heart wall in Limulus and Decapods consists of 

 several strata of striped muscle within a connective envelope, 

 and at the orifices are passive valves. The heart is held in 

 its place by elastic processes and radial musculi alares, which, 

 by their contraction, dilate the vessel. In some Copepods 

 the heart beats loo times per minute. 



Breathing may take place by the surface (Cope- 

 poda), by modified lamellary feet (Phyllopoda), or 

 appendages of the feet, by pouch-like appendages of 

 the thoracic segments (Amphipoda), or independent 

 organs attached to the base of the feet, or by the 

 mantle or lateral duplicatures of the skin in Ento- 

 mostraca. 



In Cirripedes the inner wall of this mantle is often folded 

 into gill-like lamellae between the wall and the base of the 

 cavity. In Lepadidse the hollow of these gill-folds becomes, 

 a brood-pouch. Others have gills variable in number and 

 position, at the base of the cirri. Bran- p. 



chipus and Artemia have gills on all 

 their feet. Apus on the anterior swim- 

 ming feet. In Nebalia the respiratory 

 and non-respiratory feet are sharply dis- 

 tinguished from each other; in others 

 there is not much difference. In Isopods 

 they are attached to the five abdominal 

 feet as tile-like plates, of which, in J^Tt^:^^:^^^^ 

 Sphseroma, &c., onepairis differentiated "'j^eT^ ^''"°'"'' *' ^'"^' 

 and folded, or divided into strips (lone). 

 In Oniscus and Porcellio one pair is altered into gill covers, 

 or the limbs of the last segment may form a cover (Idothaea) ; 

 others are altered into vibratile organs which create cur- 

 rents in the gill cavity. In the terrestrial Porcellio and 

 Armadillidium the anterior gill is air-holding. In Tylus four 



