HOMOLOGIES AMONG CRUSTACEA. 113 
peculiarities we may call the most important character- 
istics of the Arthropoda. Animals not having these 
marks must be classified under other sub-kingdoms. 
In Chapter I we have classified all animals into eight 
sub-kingdoms. The sub-kingdom Arthropoda includes 
probably half of the animals of the earth, the Insecta 
alone having more than half a million species. Nat- 
uralists are not agreed yet as to the number of classes 
properly belonging to the Arthropoda, but usually 
animals like the shrimp and crayfish are called Crus- 
tacea, those like the lithobius Myriapoda, those like 
the spider Arachnida, and those like the grasshopper 
Insecta. The Insects have already been described. 
The Arachnida have eight legs, simple eyes, and a 
cephalothorax and abdomen. 
The Myriapoda have many segments with at least 
one pair of jointed legs for each segment. 
The Crustacea breathe by gills throughout life. 
They pass through the nauplius stage in the course of 
development. 
Further study shows other marks for identifying 
these classes. Many exceptional forms are found and 
great patience and care are necessary in order to 
classify accurately. Nevertheless, it is profitable prac- 
tice to attempt to classify animals as we see them, even 
if we do it somewhat roughly at first. 
Laboratory Exercise for Review. A variety of 
forms kept separately in small numbered boxes and 
bottles may be used. Write in the note-book: 
(1) The number of the specimen. 
(2) Its mode of locomotion. 
(3) Its habitat, inferred from its structure and its 
resemblance to forms already known, with the reason 
for your answer. 
(4) Its food and its mode of procuring it, inferred 
from an examination of its mouth-parts, from other 
points of structure, and from resemblances to forms 
already known, with the reason for your answer, 
