The Development of Phascolosoma. 119 



The youngest stage in the development which I have observed, 

 however, is found in a larva of Fli. gouldii of sixtj'-five hours, in 

 which the coelom is already formed, and a layer of somatic mesoderm 

 covers the inner side of the body wall (Fig. 91, 92). A pair of 

 ingrowths, probably of ectoderm, appear in about the middle of the 

 body, one on each side of the ventral nerve cord, and, as in the 

 adult, nearly opposite the anus. A cavity has already appeared in 

 this mass of cells (Fig. 92, 93), and upon it lie certain cells which 

 are unquestionably mesodermal, and which form a part of the 

 coelomic epithelium. 



The nephrostome is soon established (85—87 hours in Ph. gouldii. 

 Fig. 94), as a channel which passes through the anterior side of the 

 base of the rudiment of the nephridium, the cavity of which it 

 connects with the coelom. The walls of this passage, which later 

 become ciliated, are formed from mesoderm. A narrow tubular 

 nephridiopore becomes visible at about the time that the nephrostome 

 is formed (Fig. 94). 



Musculature. The two pairs of retractor muscles (Fig. 55—57, 

 70 — 78) are inserted in front, by the branching tips of the several 

 fibres of which each consists, into each side of the supraoesophageal 

 ganglion, and are joined to the ectoderm near the posterior end of 

 the body by similar terminations. The anterior attachments of the 

 dorsal and ventral muscles of each side are close together; the 

 posterior attachments are widely separated. The points of origin of 

 the ventral muscles, at the rear, are close to the posterior extremity 

 of the nerve cord, whereas the dorsal pair are attached slightly in front 

 of the others and, on each side of the body, in about the mid-lateral 

 line. By the enormous growth at the posterior pole of the larva, 

 these points of attachment later become situated relatively nearer 

 the anterior end of the body. Each of the fibres of which the re- 

 tractors consist bears upon its side, about half way between its two 

 extremities, an elongated oval nucleus, embedded in a small mass 

 of undifferentiated protoplasm. These nuclei are distributed along 

 the whole course of the retractor, not only in the middle but also 

 at the ends. At the posterior extremity I have observed in longi- 

 tudinal sections an oval mass of nuclei, which I believe to be the 

 rudiment of still undifferentiaded myoblasts, 



One or two muscle fibres, which represent the ventral accessory 

 retractors, lie on each side of the anterior part of the nerve cord, 

 inserted in front into the posterior wall of the stomodaeum, and 



