120 John H. Gerould, 



behind into the wall of the body a little in front of the middle 

 (Fig. 98). 



Circular muscles of the body wall have the form of flat bands, 

 which are concave towards the middle of the body. Each appears 

 in cross-section like a crescentic line of fine dots, which represent 

 the constituent fibrillae (Fig. 98). Immediately beneath the circular 

 fibres are scattered longitudinal muscle fibres, which resemble the 

 fibres of the retractors, rather than those of the circular muscles, in 

 that they are spindle-shaped and not flattened. 



Fifth and Sixth Days. 



The larva at five days (Fig. 66) has a more regularly cylindrical 

 form than during the previous period. Ph. vulgare at this time is 

 about 0.5 mm, Ph. gonldii 0.42 mm in length, when fully expanded. 

 A prominent, undivided prostomium, flattened on its ventral side 

 which is covered with cilia, projects in front of the mouth. There 

 is also in both species a preoral circlet. The prostomium of the 

 larva of Sipunculus, according to Hatschek, and as I have obsei'ved 

 it in larvae of S. tessellatus at Naples, is a much less prominent 

 structure than in Phascolosoma. The buccal and anal cavities in the 

 larva under consideration are ciliated. The enteric tube has acquired 

 a lumen, which is filled with a minutely granular substance, which 

 is evidently a fluid derived from the coarser granules of yolk within 

 the surrounding endoderm cells. The body cavity is full of still 

 larger yolk granules of a grayish color, which cling together in 

 masses of various sizes (Fig. 66). 



Second Week. 



The amount of yolk suspended in the coelomic fluid decreases 

 rapidly from the sixth to the eighth day, and the larva, hitherto 

 opaque, becomes so transparent that the alimentary tube, re- 

 tractor muscles, nerve cord, nephridia, etc. become distinctly visible 

 (Fig. 70, 71). 



The simple prostomium of the five days old larva grows out at 

 the end of the first week into two lateral flat projections (Fig. 56, 

 57a, 71—73), which extend somewhat dorso-ventrally , and upon 

 which the tentacles are later to be developed. A ciliated under-lip 

 is formed beneath the mouth, simultaneously with the two lateral 

 prostomial lobes (Fig. 73). 



The body of the larva at the moment of greatest elongation now 



