50 LOWER INVERTEBRATES. 



broken away from the base and, still living, are rolled about on the bottom. Some of 

 the commercial sponges are said to be tough enough to stand this. 



The sponge is typically, or in its most perfect aspect, a vase contracted at the top. 

 In nature it has none of the usual signs of symmetry observed in other animals, and is 

 in most forms even very irregular. There is absolutely no forward or hinder end, 

 except in the embryo ; there is no right or left, except again in the embryo. Being a 

 purely sedentary animal, and having no appendages, it has become and usually is des- 

 ignated as amorphous or formless. The conditions which influence growth have 

 caused not only this degradation in symmetry, but they occasion, also, great differ- 

 ences in form in the same species. Thus, while they may be called formless in respect 

 to symmetry, from another point of view they are really animals with more forms 

 than usual. 



Among those which live near the shores and in the varied conditions of the shal- 

 low water habitats, there is the strangest diversity. Every change of bottom, every 

 change in the surrounding conditions of the current or the place to which the larva 

 may become attached, has some effect upon their aspect. Thus in the same species 

 we find flattened sheets, irregular lumps and clumps, and branching, bush-like modifica^ 

 tions of each of these in every variety, and finally vase-like shapes, either imperfect 

 and open on one side, or perfect and not wholly without grace of outline. If we pass 

 from the varied bottom of the shore-line to one of uniform character, whether the mud 

 bottoms of the deeper waters of the ocean or those nearer shore, or the sandy shallows, 

 where the surroundings and conditions of life are more uniform, we find that the 

 sponges inhabiting these localities are remarkable for greater uniformity of shape 

 within the species. 



Sponges exhibit most plainly in their forms the direct action of gravity and the 

 peculiarities of the base of attachment. In a sedentary animal the fluids of nutrition 

 would naturally tend to expend their forces primarily, in the early stages of growth at 

 the lowest points of the periphery, and after building the base, cause the sponge to 

 grow upM'ards in the direction of least resistance. This is practically what happens, 

 and if the rock is smooth and free from other animals, some species, having no heredi- 

 tary form, will grow in a broad sheet without branches ; but if the base of attachment 

 be small or crowded, the same sponge will take a bushy, plant-like outline. The force 

 of growth which otherwise would have expended itself in increasing the sponge hori- 

 zontally, is diverted by the strain on the supports or skeleton to the secreting mem- 

 branes of the threads, and we find they become thicker or denser where the strain 

 is greatest, until in some very old sponges the trunks or bases are almost solid. Above, 

 the branches are arranged so that the form is balanced, and there is the same equal 

 distribution of the weight around a central axis as in plants and in sedentary animals 

 of all kinds. This tendency or response of the animal to the attraction of gravitation 

 by equal growth in hoi-izontal planes, so as to balance one side with another, one 

 lateral organ with another, I have previously termed geomalism. Geomalism appears 

 in its primitive aspect among the sponges since they are comparatively soft and sup- 

 ported by a pliable and primitively fragmentary internal skeleton. 



It will be seen from these remarks that the form of the sponge is more largely the 

 result of the character of the base of attachment than any other cause. When this is 

 uniform, as in a mud or sandy bottom, the form is either vase-shaped or branching and 

 comparatively constant ; when upon rocks or irregular surfaces, all forms may occur. 

 Another correlation has been frequently noticed by the writer. In rapid tide-ways a 



