EMBRYOLOGY. 



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freely into this water-tube. In a somewhat more advanced pluteus, seen ob- 

 liquely from the oral side (Fig. 4.5), we can trace the mode of development of 

 the chord of vibratile cilia ; it is formed of a single continuous line, extend- 

 ing round the mouth ; it forms but a single shield, and not two, as in the 

 starfish, where the first trace of this chord is the appearance of two separate 

 arcs forming eventually two distinct plastrons. The little pluteus looks in 

 this condition like a quadrangular pyramid with a rounded apex and rounded 

 angles at the base. The corners of the base (c, e") are the origin of the first 

 arms of the pluteus (Figs. 44, 45). Owing to the great increase of the dorsal 

 and oral parts of the embryos, they change their general appearance very 

 rapidly (see Figs. 46, 47). As the intestine becomes more distinct from 

 the stomach, the angle which their axes make grows more acute (Fig. 40, 



Fig. 48. 



Fig. 49. 



Fig. 50. 



c, d) ; the mouth (m) is removed farther from the anus. The walls of the 

 oesophagus (0) are now capable of considerable expansion and contraction; 

 they are much thinner than those of the intestine or stomach. Fig. 47, 

 which is Fig. 46 seen from the oral side, shows the course of the vibratile 

 chord, the position of the arms e, e'", the great size of the rods (/•') with their 

 branches, and the difference of level between the opening of the mouth and 

 the anus. From an examination of Figs. 46 and 47 the position of the rods 

 can be determined, one main part extending from the anal extremity to the 

 arms e, another extending in a curved line (Fig. 411) from / to e", and send- 

 ing off a small branch which runs between the anus and the digestive cavity 

 (Fig. 47). This will perhaps be more clear on examining the pluteus in such 

 a way (Fig. 48) as to bring the vibratile chord into the field ; this stage does 

 not differ materially from that represented in Fig. 45, the changes which have 



