MUSEUM OF COMPARATIVE ZOOLOGY. 119 



stomach and ttie outer wall of the pluteus. The oral ciliated band is 

 not as transparent as the anal lobe of the pluteus. Stomach walls and 

 oral ciliated belt have a yellowish-green color. 



In the oldest plutei which we shall mention, figs. 21-23, the larva 

 has assumed a triangular profile when seen from the ventral side and 

 the two lateral arms, II, have pushed out on each side. The anal lobe is 

 slightly pointed ; the oral, ol, well developed, undivided, and rounded. 

 The whole external surface is ciliated. The oral band of cilia is indi- 

 cated by a closer approximation of the cells of the middle layer. The 

 distal ends of the posterior rods are pigmented. The body of the plu- 

 teus is surrounded by a superficial transparent layer of cells. The 

 mesoblastic walls of the arms are crowded with granules.* 



The rods which form the supports of the lateral arms have length- 

 ened to keep pace with the growth of the arms. These rods are not 

 latticed. The rods of the anal lobe are bow-shaped, and at the apex of 

 the anal lobe they bifurcate, the larger division extending to the apex 

 of the lobe. 



The anterior rods are smooth, and extend half way down the lobe, or 

 in some cases to the ciliated oral baud. The mouth, oesophagus, and 

 stomach are well differentiated from each other. The hypoblastic walls 

 of the latter, ga, are green and yellow. 



The oldest pluteus is a little more than tnree days old. On the 

 fourth day I left Eastport, and all my plutei died from want of care. 

 It is probable, however, that they are hardy, and can be easily raised, 

 and the young Ophiopholis traced from them to its adult. 



The following summary of the preceding observations may be 

 made : — 



1. OjoAe'ojoAo^w acM^ra^a has a development with metamorphosis, pass- 

 ing through a larval stage called the pluteus. 



2. The ova are laid in the surrounding water. The yolk has a cen- 

 tral and a peripheral region, which is distinguished in the 8-cell and 

 previous stages of segmentation. The cleavage is like that of other 

 Echiuoderms. 



3. A gastrula is formed by an mvagination of the blastoderm, and 

 consequently the stomach of the pluteus is an infolded wall of the blas- 

 toderm, and not formed by delamination from the cells in the cavity. 



4. The mesoderm cells originate in two lateral clusters. 



* MetschnikofE accurately represents, op. ck., PI. V. fig. 2, an Ophiuran pluteus 

 which has the cells " cutis " crowded in the lateral arms in the same way as in 

 Ophiopholis. 



