16 SPONGES 



Sponges are two-layered (diploblastic) Metazoa. The two layers are the 

 outer dermal layer (" ectoderm "), which consists of flattened epithelial cells 

 and pore-forming porocytes, and the inner gastral layer (" endoderm "), 

 which is composed of flagellate collar-cells or choanocytes. Between the 

 layers is a middle jelly (mesoglcea) containing wandering amoebocytes, some 

 of which form the characteristic limy or siliceous spicules of the supporting 

 skeleton. 



(a) The Dermal Layer or " Ectoderm." 



This covers the outer surface of the sponge, and is composed of flattened 

 epithelial cells, with porocytes here and there. The porocytes are cells 

 which become hollowed out and thus form the temporary inhalent pores. 

 The epithelial cells (dermal epithelium) take the form of thin scales, so 

 closely cemented together by their edges that their boundaries are not 

 readily distinguishable — i.e. they form a syncytium. In sponges which 

 have more or less complex canal systems, formed by outgrowths and foldings 

 of the body-wall, portions of the dermal layer are internal and line the 

 inhalent or incurrent canals. Groups of spicules project through the dermal 

 layer. 



(6) The Gastral Layer or " Endoderm." 



This layer lines the gastral or paragastric cavity (in the simplest sponges) 

 and the radiating chambers or radial canals (in the higher types). The cells 

 of the gastral cavity (in the higher types) are thin epithelial cells ; those of 

 the radiating chambers are collar-cells or choanocytes. The collar-cells are 

 flask-shaped ; each has a nucleus, a long whip-like flagellum projecting from 

 the free end, also a delicate collar which surrounds the lower portion of the 

 flagellum. The collar is retractile, and the flagellum also can be withdrawn. 



(c) The Middle Jelly or " Mesogloea." 



This gelatinous matrix, situated between the two layers, contains wander- 

 ing amoeboid cells or amoebocytes. The amoebocytes probably serve as 

 carriers of food and removers of waste; but some of them develop into 

 genital cells, and others become scleroblasts or spicule-forming cells. The 

 spicules are embedded in the jelly matrix, and in certain sponges are felted 

 together into a supporting skeletal framework. There are various kinds of 

 spicules, viz. : — 



(i) Monaxon spicules, having a needle-like form with a single axis, 

 (ii) Triaxon spicules, consisting of four radii of a sphere, meeting each 

 other at equal angles at the centre and thus producing a four-rayed 

 form, 

 (iii) Polyaxon spicules, which have numerous axes typically radiating 

 out from a common centre. 



Further variations occur in some sponges, giving rise to the following 

 sorts : — 



(i) Triradiate spicules, consisting of three rays branching from a common 

 centre in the form of a tripod. 



