POKIFEEA. 



215 



gelatinous Sponges or Myxospongia, is composed of horny fibres or 

 silicious or calcareous spicules. 



The horny fibres form, without exception, anastomosing networks 

 of varying degrees of thickness, and present a lamellated structure 

 (fig. 158), which indicates that they are formed of a number of layers. 

 They are formed by excretion as hardening 

 portions of sarcode. The calcareous needles 

 (fig. 159) are simple or three- and four- 

 rayed spicules, and take their origin, as 

 do the silicious structures, in the interior 

 of cells. The silicious spicules present, 

 however, an extraordinary variety of form : 

 some of them constitute a connected frame- 

 work of silicious fibres, and others are free 

 silicious bodies with simple or branched 

 central canals (fig. 160). The latter are 

 found in the form of needles, spindles, 

 cylinders, hooks, anchors, wheels, and 

 crosses, and arise in nucleated cells, pro- 

 bably as deposits round a hardening of FIG. 158. Piece of network of 

 organic matter (central fibre). 



In order to understand the morphology 

 of the Spongiaria we must begin by examining the structure of a 

 young Sponge, which proceeds from the fixed larva. The young 

 Sponge, after the formation of a ciliated gastric cavity and an ex- 

 halent opening or osculum, has the form of a simple hollow tube, 

 the walls of which are 

 pierced by pores for the 

 passage of small food 

 particles suspended in 

 the water (fig. 152). 



In this stage we can 

 distinguish three layers 

 ( 1 ) an entoderm, 

 formed of elongated 

 flagellated cells; (2) a 

 mesoderm, the skeleto- 



genous cell layer, the structure of which recalls connective tissue; 

 and (3) an ectoderm, which forms the outer layer of the Sponge, 

 and consists of a flat epithelium. The cylindrical cells of the endo- 

 derm possess at their free ends surrounding the flagellum a delicate 



horny fibres from Euspongia 

 equina. 



reous Spicules of Sycon. 



