638 ZOOLOGY. 



In the mollusks, especially the snails and cuttle-fish, the 

 gills are in close relations with the heart, so that in the cut- 

 tle-fish the auricles are called "branchial hearts." The 

 gills of crustaceans (Fig. 259) are attached either to the 

 thoracic legs or are modified abdominal feet, being broad, 

 thin, leaf-like processes, into which the blood is forced by 

 the contractions of the tubular heart. Respiration in the 

 insects goes on all over the interior of the body, the tracheal 

 tubes distributing the air so that the blood becomes oxyge- 

 nated in every part of the body, including the ends of all the 

 appendages. The gills of aquatic insects are in all cases fila- 

 mentous or leaf-like expansions of the skin permeated by 

 tracheae (Fig. 326) ; they are, therefore, not strictly homolo- 

 gous with the gills of crustaceans or of worms. 



The gills of fishes are so situated as to be constantly 

 bathed by fresh water ; in the amphibians and lung-fishes, 

 lungs, which are outgrowths of the enteric canal, replace the 

 air-sacs of the fishes, the air being now swallowed by the 

 mouth and gaining access by a special duct, the larynx, to- 

 highly specialized organs of respiration, the lungs, which 

 are situated in the thoracic cavity near the heart. 



The Nervous System. We have seen that animals of com- 

 paratively complicated structure perform their work in the 

 animal economy without any nervous system whatever. It 

 has been only recently discovered that in a few jelly-fish is 

 there, for the first time in the animal series, a consecutive 

 nervous system, with definite nerve-centres or ganglia. In 

 most Acalephs none has been found, so that the majority 

 of Cffilenterates perform their complicated movements, 

 swimming about for food, taking it in, digesting it, and re- 

 producing their kind, without the aid of what seems, when 

 we study vertebrates alone, as the most important and 

 fundamental system of organs in the body. 



The Protozoa, sponges, and most Ccelenterates depend, for 

 the power of motion, on the contractility of the protoplasm 

 of the body, whether or not separated into muscular tissue. 

 In the Hydra for the first time appear the traces of a ner- 

 vous tissue in the so-called nervo-muscular cells, one por- 



