PORIFERA. 



79 



other sponges. They present the feature of projecting into the 

 chambers without contact with their neighbours. 



It was supposed that in life their collars were everted so as to touch the 

 collars of neighbouring cells and form a continuous thin membrane lining the 

 chamber ; but this has been shown to be a post-mortem phenomenon. 



The mesoderm contains many tissue elements which are all con- 

 nected together, and are embedded in a kind of gelatinous basis. 



To revert to the language of the cell theory, those cells of this tissue, to 

 which no special function can be assigned such as reproduction, contraction, 

 skeletal formation vary somewhat in their arrangement, and so give rise to 

 different forms of mesoderm ; we may mention Collcnchyme with scattered stellate 

 cells ; Sarcenchyme gelatinous basis reduced, and granular cells closely packed ; 

 Cystenchyme with vesicular, vacuolate cells close together or separated by a 

 matrix ; Chondrenchyme like hyaline cartilage. 



There are also pigment cells, fusiform connective tissue cells, muscle cells as 

 granular fusiform cells round the 

 openings of the water-canals. 



Spicules originate in one cell, or 

 more than one cell may be asso- 

 ciated in their production (Lithis- 

 tida). It has been recently stated 

 that three cells are concerned in 

 the formation of the triradiate 

 spicules of the Calcarea, and that 

 they are derived from the ectoderm 

 (Minchin). 



The ectoderm is composed 

 of flattened cells, though the 

 cell limits are not always dis- 

 cernible. In some sponges at 

 least the cells of the ectoderm 

 seem to be capable of assum- 

 ing, when contracted, a mush- 

 room shape. 



The ingestion* of food and foreign bodies by sponges is effected by 

 the collared cells, and probably also by the flat cells of the inhalent 

 and exhalent canals, and of the surface ectoderm of the body. From 

 these the solid bodies pass into the subjacent mesoderm cells, this 

 transit being probably effected by protoplasmic flow along the strands 

 connecting the surface epithelia with the subjacent tissues, in very 

 much the same manner as food passes along the pseudopodial network 



* Fide E. Metschnikoff, " Spongiologische Studien." Z. f. w. Z., Bd. xxxii. 

 p. 372. A. Dendy, "Studies on the Comparative Anatomy of Sponges." 

 Q. J. M. S., vol. xxxv. p. 216. 



FIG. 69. Section of Corticium candelabrum (after 

 F. E. Sch.). Gk flagellated chamber, with 

 prosodus leading to it and aphodus from it. 



