214 John H. Gerould, 



a dorso-lateral position (Fig. 96 n'mu. d. I). A muscle fibre extends 

 from the tapering base of each forward through the coelom past the 

 retractor muscles into the side of the supraoesophageal ganglion not 

 far from the eye-spot. The presence in the trunk of neuromuscular 

 rudiments in a primitive state of development strongly corroborates 

 the conclusion that the retractor muscles arise from the ectoderm, 

 and not from cells of the mesoblast bands, as Hatschek believed to 

 be the case in Sipunculus. 



The first sign of muscular activity which I have observed in 

 Ph. gouldii was a twitching of the retractors in the elongated 

 trochophore of thirty-eight hours (Fig. 64). In the top-shaped trocho- 

 phore of Ph. vulgare at 44 - 45 hours (Fig. 50) similar movements 

 in the region of the stomodaeum were observed, due, as I have 

 already suggested, to the contraction of the accessory retractors. 

 The muscles thus begin to act before the yolk membrane is shed. 

 The entire prototrochal and apical region becomes drawn backward 

 by the repeated spasmodic twitchings, at first imperfectly and only 

 a little way. Presently, as soon as the coelom is established, 

 complete introversion of this part of the body repeatedly takes place. 

 The circular muscles begin to act as soon as the yolk membrane 

 has been shed. The contraction of the middle sphincter gives rise 

 to the transitory "wasp-waisted" form, to which I have already 

 alluded. Hatschek has noted in the corresponding stage in Sipunculus 

 a similar appearance. 



The reader is now in a position to consider the more funda- 

 mental changes which are involved in the metamorphosis viz. (1) the 

 establishment of the coelom, (2) the dissolution of the huge prototroch 

 cells, which is accompanied by (3) the gradual overgrowth of the 

 prototrochal region by the smaller cells of the surrounding ectoderm. 

 (1) Establishment of the Coelom. The mesoblast bands, 

 which lie on each side of the archenteron at the time of the shedding 

 of the egg membrane (Fig. 87), show clearly in Ph. gouldii a division 

 into three or four distinct mesoblastic segments, which lie immediately 

 behind the prototroch. Behind them the mesoderm forms a solid 

 undiiferentiated mass. The nerve cord at this time shows a segmen- 

 tation into a corresponding number of rounded cell groups (Fig. 86), 

 which are to be regarded as ganglia, though I have found no trace 

 of them in the nerve cord of the older larva. This metamerism is 

 probably rendered more evident at this time by the loosening of 

 the tissues, produced by the bursting of the egg membrane. It is 



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