1890-91.] The S tut dure aud Life-History of a Sponge. 435 



cup-like cavities, and with their circle of flagella all pointing 

 inwards, and driving forwards the currents of water till they 

 are finally discharged by the oscula. The " crunib-of-bread 

 sponge," already referred to, shows these flagellated chambers 

 particularly well when a small piece of the sponge is taken 

 from its native rock-pool and immediately plunged in osmic 

 acid in order to be hardened, and then sectioned. This 

 sponge also shows the incurrent and excurrent streams of 

 water very clearly, even in the rock-pool by careful observa- 

 tion, or under the microscope in a zoophyte-trough or watch- 

 glass among a little sea-water. The flagellated cells have a 

 striking structural resemblance to certain monads known as 

 the collared flagellate infusoria, as was first pointed out by 

 Professor James Clark in 1866. So close is the resemblance 

 that, according to Professor Sollas of Dublin University, they in 

 all probability " feed in the same way, and we may con- 

 sequently describe the feeding of the sponge-cell after that 

 of the infusorian. The flagellum of each sponge-cell creates 

 currents in the water towards itself, and the floating particles 

 borne along with these come in contact with, and adhere to, 

 a delicate film which surrounds the long neck of the cell like 

 a collar ; the protoplasm of the collar is in a state of active 

 circulation, streaming up one side and down the other like 

 an endless band ; the adherent food-particles are thus carried 

 by it to its base, where they come in contact with the neck, 

 sink into its substance, and find their way into the basal 

 part of the cell." ^ Here, according to Professor Sollas, digestion 

 is carried on, and the waste residue is ultimately eliminated 

 by the outflowing currents. An interesting discovery has 

 recently been made, in connection with the flagellate cells, 

 in the presence of a fenestrated or porous membrane uniting 

 the rims or collars in a circle, and serving, it is believed, as 

 a filter or trap to catch food-particles. The membrane in 

 question has been named " Sollas's membrane," after its 

 discoverer. 



It has been already stated that the mesoderm or middle layer 



of a sponge is largely made up of a mass of amceba-like cells : 



we have here, therefore, the unique occurrence of two distinct 



forms of life combining together to construct, as it were, a 



1 Cassell's 'Natural Histoiy,' vol. vi. p. 315— "The Group Spongife." 



