130 BULLETIN OF THE 



which was thought to be the infolding to form the proctodseum of the 

 last stages of the gastrula. The epiblastic cells, although becoming 

 thinner by the growth of the mesoblast, are still large and prominent, 

 while the hypoblastic are still cylindrical in shape. Both are ciliated. 

 The former layer furnishes the motor cilia of the body ; the latter the 

 ciliated lining of the stomach. 



Immediately after the first stage in the formation of the calcareous 

 spicules in the larva, which from now on ceases to be a gastrula, the 

 anterior lobe, al, and the two prominences which form later the poste- 

 rior arms, pr, begin to push out, and the region in which they form has 

 a resemblance to the three legs of a tripod. The larva when seen from 

 the oral or ventral side, PI. V. fig. 5, has on each side of the mouth, in 

 a plane in advance of this opening, a small prominence, pr, thickly pig- 

 mented, especially on its distal end, into which extends a rod from the 

 stellate calcareous rod of each side. In the interval between these rods 

 a large undivided lobe, ol; bearing the mouth, appears. This undivided 

 lobe is the anterior or oral lobe, and on its ventral surface is a circular 

 ciliated opening, or, the mouth. As this lobe grows, the mouth opening 

 is carried up with it on its side. When seen from one side, so that the 

 length of the two prominences, posterior arms, and oral lobe can be 

 compared, it will be found that the anterior lobe is more prominent 

 than the two posterior arms. In this stage the body of the larva is 

 nearly spherical in form, and as it rests on a tripod formed of the two 

 posterior arms and the single anterior lobe, the intervals between these 

 prominences are easily seen. The anal pole of the larva is pigmented, 

 and filled with numerous amoeboid and mesoblastic cells. When seen 

 from the oral pole, we notice that the ventro-dorsal diameter is about 

 the same as the lateral. The mesoblast is much thicker than either 

 the hypoblast or the epiblast. A broad band of cilia surrounds the 

 mouth. 



In a pluteus two days old, PI. V. fig. 6, raised from the eg^, we have 

 the two posterior rods still more developed than before, while the ante- 

 rior lobe is still undivided. Seen from the ventral side the distal ends 

 of the two posterior rods diverge in a V-shape, while the posterior face 

 of the anterior lobe appears rectangular. The opening of mouth and 

 anus are well seen. 



In the interior of the pluteus we notice that the calcareous rods 

 which support the posterior arms are double, and have not joined to 

 become latticed. From a point in the body of the pluteus on a level 

 with the anus these rods join the system of rods of the body. One 



