ORDINAR Y FUNCTIONS. 1 1 7 



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

 without flagella. 



The mesogloea 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 embedded in a little jelly ; 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 con- 

 nected internally with multipolar (ganglion ?) cells ; and 

 lastly, the reproductive cells, which are connected by transi- 

 tional forms with the ordinary phagocytes. 



(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 is formed 

 within a single cell, and may be speculatively 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. 



Ordinary functions. — Excepting the fresh-water Spong- 

 illidse, all Sponges are marine, occurring from between 

 tide marks to great depths. After embryonic life is 

 past, they live moored to rocks, shells, seaweeds, and 

 the like. Their motor activity is almost completely 

 restricted to the lashing movements of the flagella, 

 the migrations of the phagocytes, and the contraction 

 of muscular mesoglceal cells, especially around the exhalant 

 apertures. In the closure of the inhalant pores, sponges 

 show sensitiveness to injurious influences, but how far this 

 is localised in specialised cells is uncertain. 



