1876] 
have been employed, since there is a lack of common names, to 
aid in explaining the homologies of the various parts. 
At the first glance the lobster is seen to consist of a large an- 
terior portion, bearing the pincers and various other appendages ; 
a and a posterior jointed portion, the abdomen. Taking the third 
l segment (somite) of the abdomen as a starting-point, it is seen to 
= Consist of an upper (tergal) and an under (sternal) portion. The 
4 
: 
: 
The Lobster ; Its Structure and History. 397 
portion of the segment between these, two is called the pleurum. 
On the under side, inserted between the sternum and pleurum, is 
seen a pair of appendages, the swimmerets (pleopoda). Each 
consists of a basal portion (protopodite) and two oval fringed 
paddles, of which the inner is the endopodite and the outer the 
exopodite. This we will consider as our typical segment, that is, 
we will try to show that all the other somites are formed on the 
same plan, with more or less variation of detail. In the sixth 
abdominal segment these swimmerets will be seen to be greatly 
enlarged, and the exopodite has a transverse joint. The seventh 
or last segment (telson) has no appendages. Together with the 
Pleopoda of the sixth it forms a paddle by which the animal is 
enabled to propel itself backward in the manner familiar to all 
who have ever observed these animals when alive. The pleopoda 
of the first abdominal somite are also modified ; in the female 
they are small, slender, and soft; in the male each consists of a 
hard protopodite and endopodite, the exopodite being absent. 
The endopodite of the second segment in the male lobster bears 
an additional joint on the inner side. 
e now come to the anterior portion of the body, in which 
the Segments are not so distinctly marked. It is covered with a 
large dorsal shield, the carapace. In front it is produced, be- 
tween the eyes, into a long spine, the rostrum. In the middle — 
is à gtoove which indicates the position of the heart in the in- 
terior, It certainly does not indicate the line of separation of 
the head and thorax. 
We are enabled to separate the anterior portion into somites 
m the fact that for every pair of jointed appendages there is 
: corresponding segment. Proceeding forward from the abdo- 
com with our homologies, we first find the fifth or last thoracic 
a 4 b, Which differs greatly at first sight from the swimmerets. 
<a of a protopodite and a jointed endopodite ; in its 
3 m stages it had also an exopodite. With the thoracic limbs 
of x feature appears; to the limb or the sternum at the base 
e limb are attached one or more conical appendages consist- 
R 
my 
