THE PROSOMATIC SEGMENTS OF AMMOCGETES 305 
transformation, but throughout the Ammoccetes stage remain in 
almost as rudimentary a condition as the tentacular muscles. 
Another difficulty with respect to the tentacles is the determina- 
tion of the number of them, owing to the fact that in addition 
to what may be called well-defined tentacles a large number of 
smaller tactile projections are found on the surface of the upper lip, 
as is seen in Fig. 115. In the very young condition, 7 or 8 mm. in 
length, it is easier to make sure on this point. At this stage they 
may be spoken of as arranged in two groups: an anterior small 
group and a posterior larger group. The anterior group consists of 
a pair of very small tentacles and a very small median tentacle, all 
three situated quite dorsally in the front part of the upper lip. The 
posterior group, which is separate from the anterior, consists of five 
pairs of much larger tentacles, the most ventral pair in the mid-line 
ventrally on the lower lip being fused together to form the large 
ventral median tentacle or tongue already mentioned. This pair, 
according to Shipley, is markedly larger than the others. There are, 
therefore, five conspicuous tentacles on each side, and in front of 
them a smaller pair and a small median dorsal one. In the very 
young condition the accessory projections above-mentioned are not 
present, or at all events are not conspicuous, and the tentacles are 
also markedly larger in comparison to the size of the animal than 
in the older condition, where they have distinctly dwindled. 
This posterior group of five conspicuous tentacles is the one which 
I suggest represents the four endognaths and one ectognath. What 
the significance of the small anterior group is, I know not. It is pos- 
sible that the chelicere are represented here, for they are situated 
distinctly anterior to the other group; I know, however, of no sign of 
a markedly separate innervation to these most dorsal tentacles such 
as I should have expected to find if they represented the chelicere. 
The muscles of the upper lip, which distinctly belong to the 
visceral and not to the somatic musculature, form part of the fore- 
most segments, and in these muscles the tentacular nerve reaches its 
final destination. From their innervation, then, they must have 
belonged to the same appendages as the tentacles supplied by the 
tentacular nerve, ¢.¢. to the endognaths. What conclusion can we 
form as to the probable origin of the upper lip of Ammoccetes ? 
Since the oral chamber was formed by the forward growth of the 
metastoma, t.¢. the lower lip of Ammoccetes, it follows that the upper 
x 
