120 PORIFERA—SPONGES. 



The mesogltxa contains very varied elements, and illus- 

 trates the beginnings of different kinds of tissue. Thus 

 there are migrant amceboid 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 Pachymatisjua ; 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, which are connected by transitional 

 forms with the ordinary phagocytes. 



(c) 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," Prof. 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 

 mesogkea cells known as spongioblasts. 



Ordinary Functions. 



Excepting the fresh water Spongillidre, 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 con- 

 traction of muscular mesoglttal cells, especially around the 

 exhalent apertures. In the closure of the inhalent pores, 

 sponges show sensitiveness to injurious influences, but how 

 far this is localised in specialised cells is uncertain. 



