50 Sponges. 



Annelifls often live on and in it, without hurting it. It would 

 however, appear to possess an inherent vitality, as fragments 

 separated voluntarily or involuntarily, immediately begin to move, 

 apparently by a contractile power, as no cilia have been found on 

 them. These pieces have no fixed shape, indeed it has been 

 suggested that the common Amoebae are merely pieces of Sarcode. 

 When these fragments meet a small fixed object they divide to pass 

 it, and then reunite. After a short time this power of motion ceases; 

 the Sarcode settles on a rock, and turns into a perfect sponge. 

 Microscopically, there is no difference between the mucous mem- 

 brane from the stomach of a mouse and Sarcode, We find some 

 form of one of these substances all through the animal kingdom, 

 and always as the seat of vital power ; it has been suggested that 

 it exists in the nerves, as the substance which fills the tubes ; in 

 the Amoebae it is extremely diffused, in the nerves, very condensed. 



In the sponge Sarcode performs the digestive functions by 

 absorbing the nutrient particles brought to it by the water, which 

 may generally be seen embedded in it. 



The whole sponge is covered externally by the extremely thin 

 Dermal Membrane. In this may be perceived numerous minute 

 pores, through which the water is introduced by cilia into canals, 

 ramifying like arteries. From these incurrent canals it passes into 

 excurrent canals, like veins, which end in several principal tubes, 

 opening by " oscula.^'' Through these the particles not required 

 are carried by a stream, which, when the sponge is placed close 

 to the surface of the water, rises in an incessant fountain. 

 Professor Grant, who made this observation, placed two sponges a 

 shoi't distance from one another, when they were like two batteries, 

 keeping up an incessant fire. The pores open voluntarily, though 

 no muscles have been discovered. When closed, they leave not 

 a sign of their existence. If we wish to examine them in a dead 

 specimen, we must starve the animal to death, when the pores 

 remain open, to catch any morsel of food. If the dermal mem- 

 brane of one animal touch that of another of the same species, 

 the two individuals instantly unite ; but no pressure will unite 

 distinct species. 



Reproduction is of three kinds; (1.) by ova; (a) with an 

 ovary, (fi) without an ovary ; (2.) By gemmules, produced {a) 

 externally, {b) internally ; (3.) By fissiparity, or division of 

 sarcode. The first method is uninteresting; the third we have 

 mentioned ; there remains that by gemmides. 



These are produced generally from the interior, sometimes 

 from the exterior of the sponge. They are about one-eighth inch 

 long, shaped like a pear. The young one can move by means of 

 cilia, a power denied to the parent. Dr. Grant says, they are 



