58 THE ANATOMY OF IX VERTEBRATED ANIMALS. 



lymphatic lacunae. Valves make their appearance in the lym- 

 phatics and in the veins, and the heart becomes subdivided in 

 such a manner as to bring about a more and more complete 

 separation of the systemic circulatory apparatus from that 

 which supplies the respiratory organs. 



The Respiratory System. — In the lower Metazoa respira- 

 tion is effected by the general surface of the body. In the 

 Annelids, processes of the integument, which are sometimes 

 branched and usually are abundantly ciliated and supplied 

 with pseud-haemal vessels, give rise to branch ice. Branchiae 

 abundantly supplied with blood-vessels, but never ciliated, 

 attain a great development in the Crustacea. The access of 

 fresh water to them is secured by their attachment to some 

 of the limbs ; and, in the higher Crustaceans, one of the ap- 

 pendages, the second maxilla, serves as an accessory organ 

 of respiration. Although especially adapted for aquatic res- 

 piration, they are converted into air-breathing organs in the 

 laud-crabs, being protected and kept moist in a large cham- 

 ber formed by the carapace. 



In some mollusks (e. g., Pteropoda\ the delicate lining 

 membrane of the pallial cavity serves as the respiratory 

 organ ; but, in most, branched or laminated processes of the 

 body give rise to distinct branchiae. The mantle becomes an 

 accessory organ of respiration, being so modified as to direct, 

 or to cause, the flow of currents of water over the branchiae 

 contained in its cavity. In many adult urodele Amphibia 

 (Perennibranchiatci), and in the embrj'onic condition of all 

 Amphibia and of manv fishes, branchiae of a similar character, 

 abundantly supplied with blood-vessels, are attached to more 

 or fewer of the visceral arches. 



In all these cases the branchiae are external, and are de- 

 veloped from the integument. In Crustaceans and Mollusks 

 the blood with which they are supplied is returning to the 

 heart ; while, in the Vertebrata mentioned, it is flowing from 

 the heart ; and it will be observed that the gradual per- 

 fectioning of the respiratory machinery consists, first, in the 

 outgrowth of parts of the integument specially adapted to 

 subserve the interchange between the gases contained in the 

 blcod and those in the surrounding medium ; secondly, in the 

 increase of the surface of the branchiae, so as to enable them 

 to do their work more rapidly ; thirdly, in the development 

 of accessory organs, by which the flow of water over the 

 branchiae is rendered definite and constant, and may be in- 



