316 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Respiratory oxygen-carrying substances are commonly present 

 in the blood, as, for example, hsemocyanin in Crayfish and 

 Snail; haemoglobin in Earthworm, Dogfish, Frog, Pigeon, and 

 Rabbit. In the Crayfish water is renewed in the gill-chamber 

 mainly by the action of the scaphognathite, and in the Mussel 

 the same end is effected by ciliary action. Water streams 

 in at the mouth, and out through the branchial clefts in 

 Amphioxus and Dogfish, partly as a result of ciliary action in 

 the former, where, too, matters are complicated by the presence 

 of an atrial cavity. In the four air-breathing forms inspiration 

 and expiration may be distinguished, and special provisions are 

 made for these the muscular floor of the Snail's lung the 

 buccal respiration of the Frog the contraction and elastic ex- 

 pansion of the body-walls in the Pigeon and costal, together 

 with diaphragmatic respiration, in the Rabbit. An increase of 

 surface is gained in various ways both in the case of lungs 

 and gills. 



(2) Excretion. The contractile vacuole assists this in Amreba 

 and Vorticella. Ascaris possesses two lateral excretory tubes 

 opening anteriorly by a common ventral pore. The branched 

 excretory system of Tapeworm and Fluke is very complicated, 

 and almost every segment in Earthworm and Leech possesses its 

 own pair of complex glandular nephridia. A pair of glandular 

 kidneys (? nephridia) are found in Mussel, and one (another 

 example of asymmetry) in the Snail. The renal function is per- 

 formed in the Crayfish by the paired green glands. Amphioxus 

 appears to possess various excretory structures. The kidneys 

 of the Dogfish, Frog, Pigeon, and Rabbit are very complicated, 

 but essentially consist of glandular tubules, each commencing 

 in a filtering apparatus (Malpighian body). The kidney in 

 the Pigeon and Rabbit is a metanephros, not morphologically 

 equivalent to the kidney of the Frog, but it is preceded in 

 development by a transitory mesonephros, which is homologous. 

 The kidney of the Dogfish is partly mesonephros, partly meta- 

 nephros. In all these forms the mesonephros is at first made 

 up of tubules in which a segmental arrangement can be detected. 

 If the nephridia on each side of the Earthworm communicated 

 with a duct which opened into the posterior part of the gut, a 

 structure would be produced of similar nature. A urinary 

 bladder is present in the Frog and Rabbit, which in one case is 



