MUSEUM OF COMPARATIVE ZOOLOGY. 143 



ret.). The insertion is in the muscularis of the polypide (Fig. 56), but 

 I have not been able to determine the precise relation between the 

 muscle fibres of the great ccelomic muscles and those of the muscu- 

 laris. A comparison of Figures 44, 59, and 56 shows quite plainly that 

 both the retractor and the rotator muscles originate from a common 

 mass of muscle cells, and become distinct from one another by a 

 separation of their points of attachment to the polypide. The re- 

 tractor muscles (rnu. ret.) are attached to the oesophagus immediately 

 below the ganglion (Plate IX. Fig. 78) ; the rotator muscles {mu. rot.), 

 on the contrary, to the lateral walls of the opening leading from the 

 ccenoccel (coen.) to the cavity of the lophophore arms. These two re- 

 gions are near to each other in the young polypide, but become con- 

 stantly more widely separated with the growth of the lophophore. 

 Compare Figure 78 with Figures 74 (Plate VIII.) and 51 (Plate V.), 

 which are younger stages, cut somewhat above the level of Figure 78, 

 and more than twice as highly magnified. 



I have been able to obtain in thick sections various stages in the 

 development of the muscle fibres, some of which are shown in Figures 89 

 to 92 (Plate X.). In the earlier stages, all parts of the muscle cell stain 

 uniformly in cochineal. Later, the cell body becomes differentiated into 

 two portions, easily distinguishable by their different receptivity to the 

 dye. The more retractile portion becomes greatly elongated, highly 

 refractive, and incapable of being stained. A mass of indifferent pro- 

 toplasm, including the nucleus, still remains stainable (Fig. 90). The 

 undifferentiated portion continues to diminish relatively to the whole 

 mass of the cell, which has greatly increased in size, until little remains 

 but the nucleus, placed on one side of the muscle fibre (Figs. 91, 92). 

 Figure 92 is one of the retractor muscle fibres, in a partly contracted 

 state. The end placed uppermost in the figure was that which abutted 

 upon the muscularis of the oesophagus. Its more intimate relation to 

 the muscularis could not be traced. 



8. Origin and Development of the Parieto-vaginal Muscles. — These 

 consist of two sets, the lower, or posterior, and the upper, or anterior. 

 The posterior arise earlier. At about the time when the neck of the 

 polypide begins to disintegrate in order that the polypide may become 

 extrusible, a disturbance is seen in the cells of the outer layer of the 

 kamptoderm immediately below the neck of the polypide, and in the 

 ccelomic epithelium opposite to them (Plate XI. Fig. 97, mu. inf.). As 

 a result, several cells of each layer become organically connected with 

 those of the opposite layer, and give rise to muscle cells. A later stage of 



