300 UVLSO.V. [\-0L. I.\. 



become confined to the anterior (or upper in rotation) pole, the 

 larva ceases to rotate and applies itself to the dish obliquely, 

 that is in the plane x-y of Fig. 27. It then flattens out at its 

 spicular pole, and in this stage is shown in Fig. 28. The 

 flattening out continues and the patch of columnar ectoderm 

 grows smaller, until the young sponge has assumed a flat cake- 

 like shape. In this condition it is approximately circular in 

 outline (see PI. XVIII, Fig. 55, surface view of recently attached 

 sponge), and is entirely covered with a flat epithelium, and is 

 practically solid. The straight spicules, which in the swimming 

 larva formed a loose bundle at the posterior end, become dis- 

 tributed during the flattening of the sponge, through all quar- 

 ters of the body. After the flattening is completed, as is 

 shown in Fig. 55, the spicules project slightly all over the 

 upper surface. The outline of the sponge soon becomes irreg- 

 ular, and the body undergoes many changes of shape, which, 

 however, are so slow and gradual as to escape notice, unless 

 drawings of the outline are made at interv^als. In the solid 

 body of the sponge the canals and flagellated chambers appear 

 as separate cavities, which subsequently unite with one another ; 

 and the pores and oscula make their appearance as simple 

 perforations of the outer skin. All the essential features of 

 the sponge body are established two or three days after attach- 

 ment. At this time the area of the body is considerably 

 greater than that of the swimming larva, but its actual bulk 

 cannot much exceed that of the latter. After this stage, prac- 

 tically no growth occurred in the sponges I kept. They lived 

 for weeks, but whether from lack of proper food or for some 

 other cause, they did not continue to develop. 



It sometimes happens that a larva attaches to the surface 

 film of the water. In this case fixation takes place at the non- 

 spicular pole, which flattens out to form a wide surface of 

 attachment (see PI. XVII, Fig. 37, vertical section through a 

 larva so attached). The columnar ectoderm in such larvae 

 metamorphoses in a different fashion from that ordinarily fol- 

 lowed, in that the ectoderm over the surface of attachment 

 becomes flat, while that on the sides is still columnar, as may 

 be seen in Fig. ^j. Of the larvae that attached in this way. 



