170 Macrteay Memoriat Vouume. 
3. Muscles of the tail fin (figs. 8 and 9). 
The numerous muscles of the tail fin may be arranged in four groups as follows :— 
a. Axial muscles of the telson. These have been already described (p. 167). 
b. Extrinsic muscles of the lateral tail-lobes or appendages of the sixth 
abdominal segment: these are divisible into flexors and extensors. 
e. Adductor muscles of the tail fin, passing between the telson and the 
lateral tail-lobes and serving to approximate the various parts of the 
tail fin during the back-stroke. 
d. Muscles of the endo- and exopodites. 
Flexor of the lateral tail-lobe. There are no fewer than five well-marked muscular 
bands serving to depress each lateral tail-lobe, but as they are all inserted by a single 
tendon they are best considered as five heads of one and the same muscle. 
The dorsal head is a conical muscle arising from the middle region of the sixth 
abdominal tergum: the vertral head is a large flat band arising from the fifth sternum 
and passing backwards and outwards so as largely to cover the longitudinal flexor of 
the sixth segment m a view from beneath: the posterior head arises from the sixth 
sternum : the external head arises from the fifth sternum laterad of the ventral head: 
the accessory head is a branch of the flexor of the telson. All five heads unite in a 
strong tendon which is inserted into the proximal edge of the basipodite near the 
middle of its ventral side. 
The dorsal extensor of the lateral tarl-lobe arises from the sixth tergum imme- 
diately caudad of the dorsal head of the flexor, and is inserted by a distinct tendon 
into the dorsal surface of the proximal edge of the basipodite. 
The external extensor of the lateral tail-lobe arises from the sixth pleuron and 
passes directly backwards to be inserted by a distinct tendon immediately in front of 
the preceding. 
The long adductor of the tail fin arises by two heads from the tergum of the 
telson ; the origin is an extensive one, occupying about the anterior half of the 
tergum. The muscle is inserted into the arthrodial membrane uniting the basipodite 
of the lateral tail-lobe with the sternum of the sixth abdominal segment. 
The muddle adductor of the tail fin arises from the tergum of the telson 
immediately in front of the inner head of the preceding ; it is inserted into the flexor 
tendon of the lateral tail-lobe. 
