THE DEVELOPMENT OF SYMBliANCHHS j\[ARMORATUS. 15 
plexus begins to differentiate in Stage 28, the plexus being 
somewhat diffuse and wide spreading at first. 
A reconstruction of Stage 30 shows that the fused spinal 
ganglia of nerves 1 and 2 forms a large structure just outside 
the occipital arch. It also shows that the shoulder girdle 
has suffered a backward migration relative to the spinal nerves, 
for nerves 1 and 2, now fused into one nerve, and nerve 3, 
to a less extent, have to run backwards more or less parallel 
to the vertebral column to join the brachial plexus. This 
backward displacement of the shoulder girdle has resulted in 
Stage 33 in the further elongation of the nerves running to 
the brachial plexus. A small branch of spinal nerve 4 joins 
the plexus, though I have no definite developmental evidence 
that myotome 4 has played any part in muscularising the fin. 
Movements of the Fins. — As soon as they are hatched 
the larvm begin to make constant movements with their great 
vascular pectoral fins, doubtless for respiratory purposes ; 
the motion is jerky and very characteristic. Each fin is 
moved through an angle of about 180°, being flat against the 
body at the end both of its forward and backward move- 
ment, and the direction is generally opposite on the two 
sides, one fin being flapped forward while the other is being 
turned back. This produces an impression on the observer 
watching the larvae from above as if the two fins were a 
single bar pivoted through its centre to the body of the larvae 
and surging backwards and forwards in a jerky manner. 
This form of motion is not absolutely fixed, however, and 
when the larvm were disturbed they were sometimes seen to 
flex and extend their fins together, thus using them as 
ordinary locomotor organs. 
The further history of the pectoral fins is given under 
Stages 32, etc. No trace of pelvic fins has yet been forth- 
coming though a careful search has been made. 
A fuller history of the gill-chamber and the opening is 
given in connection with the development of the alimentary 
canal; here these structures will be only briefly described. 
Gill-slits such as are seen, for example, in a developing trout 
