METAZOA PORIFERA. 67 



among the most specialized calcareous sponges artd Ha- 

 lisarca (=Oscarella) among the silicious group. Up to 

 the time of the formation of the gastrula-like embryo the 

 development of the sponge is parallel and similar to that 

 of the Coelentera. After the gastrula-like stage, how- 

 ever, the transformations that the young sponge goes 

 through are peculiar to the Porifera. These stages end 

 in the formation of an oval form with a girdle of larger 

 cells and a circlet of cilia around what was the opening 

 of the gastrula-like embryo, but which has been plugged 

 up by the growth of cells in the interior. This larva is 

 collared ; it is the typical Poriferan form and when one 

 finds it he knows that he is looking at the young of a 

 sponge. This little active creature is not guided by its 

 intelligence in the search for food nor by any particular 

 instinct. The tides and currents carry it (since its own 

 power of swimming is not very effective), and where 

 they flow there is always food of the right sort in abun- 

 dance. If the little larva floats out of the proper region 

 it would fasten itself probably to any sufficiently smooth, 

 hard substance, and either lead a half-starved abbrevi- 

 ated existence, or meet with an untimely death choked by 

 muddy sediments or killed by some other equally effective 

 agency. When about to settle, the collar spreads itself 

 out by growth, forming the base, and by closely fitting 

 itself to the surface excludes the water and air, thus fas- 

 tening the body by the weight of these elements to its 

 selected spot as a boy fastens a sucking disk of wet leather 

 to a stone. 



The cavity which appears in the body after this stage 

 has no external opening ; the latter breaks through at 

 the end opposite the plugged up opening of the gastrula- 

 like embryo. The cells of its walls have flagella and 

 collars. These organs appear at different times, and 

 on different parts of the body, but they become perma- 

 nent in the interior of the ampullae or little sacs after 

 this stage and are not found, as a rule, on the membranes 

 of other parts or on the exterior. 



