THE PROSOMATIC SEGMENTS OF AMMOCCETES 305 



transformation, but throughout the Amnioccetes 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 lar^e 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 chelicerae 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 chelicerae. 



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, i.e. 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, i.e. the lower lip of Ammoccetes, it follows that the upper 



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