SPONGES. 



55 



We cannot hope to disentangle the intricate rehitions of the parts in such confused 

 structures as the sponges without studying the history of tlieir development. The 

 young can always be relied upon to present the observer with simpler or more element- 

 ary conditions, and generally help us materially in understanding and translating the 

 adult structures. 



As we have said, thr niak' and female elements are found within the sponge. After 

 fertilization, the egg undergoes a regular segmentation, and then the two ends of the 

 body become distinguisiiable, one being composed of smaller cells than tlie other. 

 Tlie embrj-o is hollow at this the so-called morula stage, but soou the central hollow, 

 the segmentation cavity of embryologists, becomes tilled in the following manner. 

 Tlie cells of one end of the embryo become pushed in, much as one inverts the finger 

 of a glove, and these constitute the inner layer or endoderm of the young sponge. 

 In this, which is called the gastrnla stage, there are then two layers. In the cal- 

 careous sponges tliey form a cup with a mouth at one end, but in the carneosponges 

 tlie gastrula is usually but not invariably solid, the invaginated endoderm comiiletely 

 filling the interior. The mesoderm is developed between these two layers, but from 

 which one is not yet known. The spicules begin to be 

 fiirnu'd in the mesoderm soon after its appearance, and 

 seem to be due to direct transformation of single cells. 



These young larva? swim rapidly through the water 

 by means of the cilia, or small hairs, which clothe the 

 exterioi", and which can be moved like so many oars 

 with force and rapidity at the will of the tiny animal. 

 The smaller end in the larva of the calcareous sponge 

 is foremost as the little creature moves aimlessly about. 

 When it encounters any obstacle it usually exhibits no 

 ability to back off, but manages by keeping its cilia in 

 constant motion to get away by rolling around the 

 olistruction. At last the embryo settles down, with its 

 mouth or blastopore below, upon the space to wliieh it 

 is to become attached. The membranes at this end 

 form a sort of sucker, which spreads itself out and enables the animal to exclude the 

 water between it and the surface to which it is being 

 applied. The pressure of the water holds the sponge 

 in its place, and on some smooth spots this may con- 

 tinue to be its only anchorage, but in rougher situa- 

 tions it naturally acquires additional hold by growing 

 into any cavities or around any projections. 



On soft, muddy ground fresh-water sjionges usually 

 begin to grow upon some small substance, which often 

 is very small, and then the weight of the growing sponge 

 may sirdv a portion of the stalk into the mud below. 

 This ]iortion tlien dies, but even when dead it ])Iays its 

 part and forms an anchor for the whole structui'e. We 

 cannot imagine an ordinary sponge growing upon a muddy surface unless the water was 

 absolutely still or the mud hard ; otherwise the tiny creature would be suffocated by 

 the sediment. The deep-water mud sponges of the sea (Hi/alonema, etc.) have, how- 

 ever, grown so long on soft bottoms that they have developed a system of threads 



'ree swiimiiiug young of 

 Sijcandra. 



I. 52 —Section of ittacUed embno 

 of '^tfcajidra a, pinnitue stomacli 

 6, blastopore e ectoderm h, endo 

 derm, .s, segmentatiou cavitj . 



