1 28 PHYL UM FOR I PER A SPONGES. 



nourished and multiply more rapidly than those of the 

 outer layer. 



(c) By infoldings of the outer layer and a subjacent 

 sheath of mesoglcea subdermal spaces may be formed ; 

 an outer cortex may be distinctly differentiated from the 

 internal region in which the flagellate chambers occur ; the 

 pores may collect into sieve-like areas, which open into 

 dome-like cavities; these and many other complications 

 are common. 



(d) The covering layer usually consists of flat epithelium, 

 but flask-shaped cells have also been observed (Bidder). 

 It may be folded inwards, as we have noticed, and, accord- 

 ing to some, it also lines the inhalant or afferent canals in 

 whole or in part. In a few cases, e.g. Oscarella lobularis, 

 it is ciliated, and its cells may also exhibit contractility, as 

 around the osculum of Ascetta clathrus^ though the con- 

 tractile elements usually belong to the mesoglcea. 



The inner layer consists typically of collared flagellate cells, 

 but in the more complex sponges these are replaced, except 

 in the flagellate chambers, by flat epithelial cells, with or 

 without flagella. 



The mesoglosa contains very varied elements, and illus- 

 trates the beginnings of different kinds of tissue. Thus 

 there are migrant amoeboid cells (phagocytes) ; irregular 

 connective tissue cells ; spindle-shaped connective tissue 

 cells, united into fibrous strands ; contractile cells, e.g. 

 those forming a sphincter around the oscula of some forms, 

 such as Pachymatisma ; skeleton-making cells ; pigment- 

 containing cells; supposed nerve cells, projecting on the 

 surface, and believed to be connected internally with 

 multipolar (ganglion?) cells; and lastly, the reproductive 

 cells. 



(e) The skeleton consists of calcareous or siliceous 

 spicules, or of spongin fibres, or of combinations of the 

 two last. A calcareous spicule is formed of calcite, with a 

 slight sheath and core of organic matter ; a siliceous spicule 

 is formed of colloid silica or opal ; the spongin is chemically 

 somewhat like silk. Uniradiate, biradiate, triradiate, quadri- 

 radiate, sexradiate, and multiradiate spicules occur, and 

 they are effective in keeping the meshes open and in giving 

 the body architectural stability. In every pole scaffolding 



