rOKIFEEA (sponges). 



45 



cells — the " sponge-particles " or " sarcoids " — which differ in their 

 characters in different parts of tlie sponge. Some of tlie sponge- 

 particles are very like Amcehce, since they consist of little masses 

 of gi-anular protoplasm, furnished with a nucleus, and capable of 

 throwing out pseudopodia (fig. 22, C), by means of which they feed. 

 In the outer layer of the sponge, these sponge -particles may be 

 arranged so as to form a kind of membrane ; and at other times 

 they become moi-e or less coalescent, giving rise to a common gelat- 

 inous matri.x, and losing their separate distinctness. Other sponge- 

 particles, again, instead of being like Amcebce, are closely similar 

 to those Flagellate Infusoria which possess a collar round the 

 flagellum. These flagellate sponge-particles (fig. 21, C, and fig. 22, B) 

 possess a nucleus and one or more contractile vesicles, and they are 

 arranged within the so-called "ciliated chambers" (fig. 21, a a) of 

 the canal-system. 



The enth-e aggregate of sponge-particles is so arranged as to be 

 traversed by a series of cjinals, which convey water in and out of 

 the organism, and are connected with respiration and the procuring 



Fig. 22.— A, Portion of Gravtia, highly magnified, showing the spicules and the 

 sponge-iiartioles. B, A single sronge-particle of Graiitia comfTesm, greatly enlarged, 

 showing the membranous collar (o), the flagellum (/), the contractile vesicles 

 (c c), and the nucleus (n)- C, A sjionge-particle of Hrrintia compressa. with the 

 ]'seudopodia ijrutruded and without the flagellum, greally enlarged. (B and C are 

 after Carter.) 



of food. Looking at the skeleton of the dried sponge, the most 

 obvious sign of the existence of this " aquiferous system " that we 

 see is the presence of one or more large openings (fig. 23, A), to- 

 gether with a vast number of much smaller apertures. These latter 



