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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



orifice of invagination, or anus (o), is situated just behind 

 the centre of the ridge, and the cilia on the anterior lobe 

 and ridge are long, while those on the posterior lobe and 

 dorsal surface are small. A number of brownish-red pig- 

 ment spots are scattered over the surface of the body. 



At each end of the ridge, which will be spoken of here- 

 after as the ciliated ridge, there is a small, transparent, 

 three-pronged spicule (d), the beginning of the calcareous 

 skeleton of the larvae. By comparing the side view with 

 the ventral view, one of the prongs of this spicule will be 



seen to point towards 

 the anterior lobe, one 

 towards the posterior 

 lobe, while the third 

 runs along the ciliated 

 ridge, towards the mid- 

 dle of the ventral sur- 

 face . 



IV. The Develop- 

 ment of the Pluteus or 

 swimming larva. 



FIG. 61. Ventral view of 

 an older larva. (Drawn from 



nature by W. K. Brooks. ) 

 FIG. 61. 



a. In the ventral view of a larva from twelve to eigh- 

 teen hours older (Fig. 61), the ciliated ridge is much 

 more marked, and projects beyond the outline of the 

 body, so that the sides of the anterior and posterior lobes 

 are concave. The posterior branch of the spicule, a, is 

 greatly lengthened, and reaches nearly to the posterior 

 end of the body, while a fourth branch has made its ap- 

 pearance, and runs towards the anterior edge of the cil- 

 iated ridsre. 



