124 Report of the Microscopical Section. [Sess. 
striation. Three distinct layers are discernible. The outer, 
called the epiostracum, is a thin tough areolated substance; 
subjacent to this is a layer called the ectostracum, which 
is made up of thin lamine traversed by delicate strie; the 
rest of the section consists of a layer termed the endostracum, 
which is also a beautifully striated substance. 
This slight glance at the external covering of the crayfish 
permits us to proceed to our examination of the internal 
organs, and the first to engage our attention is the muscular 
system. The principal muscles are—(1) the flezor abdominis 
muscle, a powerful mass extending along the lower interior — 
side of the abdomen, from near the anterior border of the © 
thorax to the base of the telson, and united in its course by a 
series of winding slips to each of the abdominal sterna. Its 
function is to bend the abdomen inwards upon itself. (2) 
The extensor abdominis muscle, a much less powerful mass, 
running along the upper interior side of the abdomen, and 
attached to cords of the abdominal terga. Its function is to 
straighten the abdomen. (3) The levator abdominis muscle, 
arising immediately above the extensor, and passing obliquely 
backwards to be inserted into the first abdominal segment. 
(4) The adductor muscles of the great forceps or chele, very | 
powerful masses, the functions of which are to close the forceps _ 
so as to enable them to grip anything; and (5) the smaller 
abductor muscles of the same, whose function is to open the — 
forceps. (6) Adductor muscles of the mandibles, strong fan- 
shaped masses passing from the hinder edge of the mandibles — 
to the cephalic shield. In addition to these, several muscles | 
pass from the stomach and antennary organ to be attached to 
the adjacent exoskeleton. 
The alimentary organs—The mouth of the crayfish is a | 
longitudinal parallel-sided opening on the sternal or under © 
side of the head. It is bounded by two lips, called respec- 
tively the labrum and metastoma, while it is protected 
externally by the powerful mandibles, and also by the two — 
pairs of maxillz, while the three pairs of maxillipedes also lie — 
adjacent to it. The mouth leads into the stomach by a wide 
opening called the cesophagus or gullet. 
The stomach is a beautiful complicated organ divided by a 
constriction into two chambers, the anterior and larger called 
