105 



merited mucilaginous coat. The first developmental processes are passed 

 very rapidly; after 6 hours the embryos were free swimming gastrulae, 

 and already at the age of 24 hours they had assumed the shape of young 

 plutei^ the skeleton having already developed its characteristic struc- 

 ture; on the other hand the intestinal tract is somewhat behind in its 

 development, the mouth having still only the form of a small round 

 opening (PI. XIII, Fig. 5). This young larva otherwise shows a charac- 

 teristic concavity on its dorsal side, as seen in PI. XIII, Fig. 6, which 



Fig. 42. Skeleton of the larva of Mellita 6-perforata. ^">/i. A. from the ventral side; 

 B. side view. Letters as in flg. 20. * The rudiment of the upper ventral transverse rod; 



vtr* lower transverse rod. 



represents the young larva in side view. They are still rather opaque, 

 and the vibratile band is not yet fully differentiated. Some red pigment 

 has been formed. After 2V2 days the larvse had assumed their full shape, 

 with both posterodorsal and preoral arms developed. At the age of five 

 days the metamorphosis was beginning, the body skeleton had been partly 

 absorbed, the large fenestrated plate in the posterior end remaining, 

 however, unaltered, evidently passing directly into the apical system of 

 the sea-urchin. Muscles were distinctly seen to connect the bases of the 

 postoral and posterodorsal rods, these four arms being thus actively 

 movable, in the same way as in the larva of Mellita 5-perforata (or testu- 

 dinaia), as described by Grave. As the metamorphosis did not proceed 



1) Caswell Grave. Some points in the structure and development of Mellita testudinata. 



Johns Hopkins Univ. Circ. No. 157. 1902. 



14 



