SOME OF THE WAYS IN WHICH ANIMALS BREATHE. 315 
a Crayfish is at rest it requires fresh supplies of oxygenated 
water. To obtain this it does what Crustacea nearly always do 
when they need a fresh organ; it makes use of one of its 
appendages (the second pair of maxille), part of which it con- 
verts into a scoop-shaped organ which, lying at the anterior end 
of the box in which the gills are placed, moves backwards and 
forwards about 200 times a minute, and by scooping out water 
at that end creates a vacuum which is filled by a rush of water 
into the hinder end of the box. 
A very interesting series of modifications are found among 
the Echinodermata; while the Starfish, in which the skeleton is 
often loosely reticulated, has no special respiratory organs, the 
Brittlestar, which has a closely-compacted skeleton, has a pair 
of clefts at the base of each of the five arms; these clefts lead 
into pouches or burse, the walls of which are thin and project 
into the body-cavity ; into these pouches water enters and again 
escapes, taking in with it a fresh supply of oxygen, and bringing 
away with it carbonic acid. Among the regular Sea-urchins the 
gills are ordinarily external, and in the dry test their position 
may be noted by the notches or indentations around the margin 
of the mouth of the test. In the most ancient extant forms— 
the Cidaride—the gills are not external, but, as Prof. Charles 
Stewart has shown, there are five internal gills around the 
Lantern of Aristotle; here the water of respiration enters 
within the boundary of the test in much the same way as it 
does in Ophiurids. 
3. Another set of breathing-organs obtains among animals 
that live on land, and in a comparatively small number of 
marine forms. Here, in place of projections outwards, there 
are depressions inwards, or in some other way cavities are 
formed into which air passes to meet perhaps with the blood 
which is richly distributed in the walls of the cavity. The best- 
known apparatus is that of lungs; as we know them in 
ourselves and all higher Vertebrates, such as other mammals, 
birds, and reptiles, these lungs are outgrowths from the anterior 
part of the digestive tract. It must not, however, be thought 
that this lung-cavity is confined to the higher Vertebrates; a 
sacular outgrowth from the intestinal tract is seen in a number 
of fishes; in some, as for example the Salmon, the air-sac, as 
the organ is then called, remains connected throughout life with 
