EMBRYOLOGY. 



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connects with the surrounding water, through the water-pore b, which has 

 pushed its way to the surface. The rods keep pace with the growth of the 

 arms (Fig. 50) ; the water-tubes have not increased in size, they are still two 

 distinct bodies. The outline of the anal part of the pluteus is quite pointed ; 

 the aboral side is regularly arched, with a slight depression at the point 

 where the water-pore opens (b, Fig. 49). The opacity of the pluteus has in- 

 creased to such an extent that it becomes impossible to define clearly the 

 outline of the water-tubes in the stages which come between Figs. 49 and 51. 

 I am unable to state positively whether the two water-tubes are united in 

 this and older embryos. All I could distinctly see was the great increase in 



Fig. 51. 



Fig. 52. 



size of the water-tubes ; but at the same time it becomes a puzzling matter 

 to trace the limits of these tubes, owing to the delicate walls which bound 

 them. Their presence can only be traced by the fine line which runs across 

 the oesophagus from each side, and by the water-pore and the tube leading 

 to it (b, Fig. 51). In a profile view of a pluteus considerably older than that 

 represented in Fig. 50, the epaulettes (v", Fig. 5l) have assumed a more inde- 

 pendent position, forming a curve somewhat similar to the arc from which 

 the median anal arms of the Brachiolaria are developed ; the third pair of 

 arms bulges out quite prominently (e IV ) when seen in profile ; the fourth pair 

 of arms is visible (/') ; the rod which eventually extends in the interior of it 

 is a straight rod (r") with a slight point in the middle, at present disconnected 

 from the remaining part of the calcareous framework. This set of rods and 



