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 
trachex (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 Colenterates 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 Coelenterates 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- 
yous tissue in the so-called nervo-muscular cells, one por- 
