HOMOLOGIES AMONG CRUSTACEA. IIt 
waves, an excellent illustration of the loss of powers 
by disuse. 
There are also many forms of fish-lice which live a 
parasitic life by attaching themselves to a portion of 
some fish and living on either the blood or food of their 
host. These bear very little resemblance to the cray- 
fish. Some have even lost the gills and breathe only 
by the external surface of the body. These fish-lice 
hatch from the egg as free-swimming larve, bearing 
a striking similarity to other crustacea at this stage 
of growth. The common larval form is called the 
nauplius. (See first stage in Fig. 95.) Ithas a single 
eye and three pairs of appendages. The higher crus- 
tacea pass through this nauplius stage before hatching 
from the egg. From this stage, the higher forms of 
crustacea which lead an active life develop more ap- 
pendages, and more acute sensibilities, with a corre- 
sponding increase of complexity in the nervous system, 
while the parasitic forms lose the eyes and locomotive 
appendages, and their whole structure degenerates into 
machinery for digestion and reproduction. 
AN EXERCISE FOR THE NOTE-BOOK. 
Rule a page in your note-book in the manner indi- 
cated on p. 112 and fill the blank spaces with such 
descriptive terms as you have learned in the previous 
lessons. 
Classification of the Arthropoda. We have already 
noted the characteristics of the class ‘‘Insecta’’ by 
selecting the points of resemblance among them. We 
readily see that the spider, the lithobius, and the cray- 
fish cannot be classed as Insecta without changing our 
present definition. We find it more convenient to 
include all the animals thus far studied in a larger class 
which naturalists have agreed to call Arthropoda. 
The Arthropoda are called a sub-kingdom or phylum 
because they constitute one of the large divisions of the 
