178 



THE ANIMAL KINGDOM 



to attached organisms), and the crab is protected from predation by the 

 disagreeable flavor ol the sponge. 



Some ol the spider crabs and other slow-moving crabs break off 

 pieces of living sponge and hold them or glue them on their backs, 

 where they may become permanently attached and grow. Such crabs 

 also plant other attached organisms on their backs, and walk about like 

 animated "gardens." They must repeat this operation each time they 

 shed their shell. 



Most sponges apparently have an unpleasant taste to most animals, 

 for only a few snails eat them. Fish avoid sponges, and hence many 

 smaller organisms seek refuge inside them. Any sizable sponge, selected 

 at random, will be found to be sheltering a number of animals in its 

 canals. 



58. Reproduction 



Sexual reproduction in the sponges, as in the protozoa, has been 

 studied in too few species to permit generalizations. All sponges studied 

 appear to be diploid, and to have the usual metazoan processes of 

 oogenesis and spermatogenesis as described in Chapter 6. Fertilization is 

 internal. The eggs are retained just beneath the choanocytes where they 

 are fertilized by sperm brought in with the current. 



The best studies of early development are in the genera Syco7i and 

 Grantia of the class Calcarea. In these the egg cleaves to form a blastula- 

 like structure (Fig. 9.6, A) that is inside out when compared with the 

 blastula stages of other animals. The nuclei lie toward the inner ends 



Materncd 

 cTi oa.no C3rte-S 



Figure 9.6. Development in the sponge, Sycon. A, The embryo lies embedded be- 

 neath the choanocytes of the parent. B, Eversion. C, Free-swimining amphiblastula. 

 D, Attachment and invagination. (A and B after Dubosq and Tuzet; C and D redrawn 

 from Hyman.) 



