STRUCTURE OF SPONGES. 125 



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, according 

 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 mesoghva 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 in 

 a general way it may be said that they are arranged so that 

 they give most architectural stability. Each spicule begins 

 to be formed by two " scleroblasts," and may be specula- 

 tively regarded as an organised excretion. " During its 

 growth," Professor Sollas says, " the spicule slowly passes 

 from the interior to the exterior of the sponge, and is finally 

 (in at least some sponges Geodia, Stelletta] cast out as an 

 effete product." The fibres of spongin are formed as the 

 secretions of mesoglcea cells, known as spongioblasts. 



