300 WILSON. Vion. Lx 
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 Pl. 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 intervals. 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 Pl. 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. 37. Of the larvae that attached in this way, 
