THE PHYLUM POR/FERA 179 



of the cells rather than the outer ends, and the flagella that appear on 

 the cells toward the animal pole project inward instead of outward. 

 The embryo is also peculiar in having a mouth at the vegetal pole 

 through which food is taken from the parent. The food is utilized by 

 the cells and in this way the embryo grows. ^Vhen fully developed the 

 embryo turns inside out (Fig. 9.6, B) through its mouth and then pene- 

 trates through the maternal choanocyte layer to escape into the chan- 

 nels of the parent sponge. The flagellated cells, whose flagella now 

 project outward, form the anterior half of the larva and the nonflagel- 

 lated cells make up the posterior portion. This free-swimming stage is 

 the amphiblastula (Fig. 9.6, C) and is similar in appearance to the 

 blastulae of a few other animals. The amphiblastula swims away and 

 attaches to the bottom by its anterior end. As it becomes attached, the 

 anterior, flagellated half invaginates into the posterior half to form a 

 two-layered structure (Fig. 9.6, D). The flagellated cells become the cho- 

 anocytes while the outer layer forms all the rest of the sponge. 



The presence of flagella that project inward and the later inversion 

 of the embryo through its mouth are unique to the sponges as features 

 of sexual reproduction. A similar process is found in the colonial flagel- 

 late, Volvox, but is associated only with asexual reproduction. 



The development of other sponges is less well known but they 

 follow different developmental patterns. Free-swimming larvae of many 

 species have been found, and in some of these a process similar to 

 gastrulation in other animals takes place. An outer flagellated layer 

 completely or partially surrounds an inner cell mass (Fig. 9.7). When 

 such larvae attach and develop, the inner cell mass produces the bulk 

 of the sponge. In some forms the flagellated cells migrate inward to 

 become the choanocytes, while in others they are destroyed and the 

 choanocytes develop from the inner mass. 



Most sponges also reproduce asexually. Pieces of some sponges fall 

 off, attach to a new substrate, and grow. In others, flagellated embryos 

 are produced that resemble the sexually produced larvae. These swim 

 away and attach. In still others, including fresh-water sponges, balls of 

 cells embedded in the body are surrounded with a capsule. After the 

 sponge dies (during the winter in fresh-water forms) and the body de- 

 cays, these gemmules are released. Many of them are equipped with 



A B 



Figure 9.7. Other sponge larvae. A, From the class Calcarea. B, From the class 

 Demospongia. 



