METAZOA PORIFERA. 83 



PI. 84, fig. 10, is a later stage still enclosed in the cap- 

 sule ; the cylindrical ectoderm cells have already devel- 

 oped whips ; the plug is strongly pigmented. In the 

 mesoderm, flint needles (colored blue in the figure) have 

 begun to form. They are at first irregular and scattered. 

 It is a fact of great significance that the spicules appear 

 before the formation of the cementing material, spongin. 

 The latter is probably a secretion of the mesoderm, and 

 is deposited according to need in layers around the 

 spicules (see PI. 84, fig. 20). This furnishes a strong 

 argument in favor of the view that a part at least of the 

 horny sponges are descendants of the silicious sponges. 

 PI. 84, fig. ii is the free-swimming larva which has 

 escaped from the capsule and the body of the parent. 

 As it swims the pointed end is directed forward. No 

 inner cavity yet exists. Figs. 12 and 13 represent the 

 larva just before becoming attached. It is now much 

 flattened (fig. 12, peripheral view; fig. 13, broadside). 

 About thirty-six hours after settlement, it looks as shown 

 in fig. 14. The isolated spicule in the larva is seen in 

 fig. 15, still lying within its cell. No cavity had appeared 

 two and a half days after settlement. Fig. 16 is a view 

 of the young sponge (natural size) five days after becom- 

 ing attached. The ampullaceous sacs with whipped cells 

 are now numerous and open into a wide cavity. The 

 cloacal opening arises on this day (the fifth) by the body 

 cavity breaking through the outside wall, and on the 

 same day and by a similar process the pores are formed 

 (see fig. 17, a vertical section of the sponge at this stage). 

 When the canals and pores appear the stream of water 

 acts effectively upon the position of the needles and 

 forms radial lines. Figs. 18 and 19 give us the external 

 and internal structure of the adult. Fig. 20 shows the 

 spicules of the adult bound together by spongin. Fig. 21 

 represents a small female colony. No. 85 is a larger 

 adult. 



The following is a summary of the time required for 

 the six stages of development, as given by Keller. 



