PORIFEKA 



19 



support (Fig. 4). The entire process of gastrulatiou and 

 attachment proceeds with uncommon rapidity. 



In the gas trula- stage ectoderm and entoderm are not 

 closely applied to each other ; but one notices between them 

 a space which must be interpreted as the remains of the 

 segmentation cavity (Fig. 4 m), and which is filled with a 

 gelatinous hyaline mass. Accoi-ding to Metschnikoff, in- 

 dividual cells of the granular ectodermal layer migrate into 

 this mass, and lead to the formation of the mesenchyma, the 

 so-called mesoderm, between the two primary layers. The 

 first skeletal structures arise in these cells in the form of 

 small rod-like needles ; triradiate 

 ones are formed later, and finally 

 quadriradiate ones. 



After the gastrula mouth has be- 

 come narrowed and finally closed, 

 the hollow body of the larva, which 

 l)as no external opening, elongates 

 in the direction of the chief axis, 

 and grows out into a cask-like or 

 cylindrical form (Fig. 5a), the 

 upper surface of which consists of 

 a thin membrane, which acquires 

 at its centre a circular opening, 

 the beginning of the exhalent 

 orifice (oscnlum, Os), which soon 

 enlarges. At the same time the 

 inhalent openings or pores (po) 

 appear as perforations in the 

 lateral walls. Since, moreover, 

 the epithelial layer of the ento- 

 derm acquires the character of 

 flagellate collar-epithelium, the characters typical of the 

 Porifera are completed in this ascon-like stage (Fig. Sa, 

 Olijnthus). The development into the Sycon takes place by 

 the radial tubes becoming established as simple evaginations 

 of the body- wall (Fig. 56) ; at first a circle of radial tubes 

 makes its appearance at about the middle of the body ; to 

 this a second is soon added, and so on. 



Fig. Sh. — Older, attached 

 stage of Sxjcandra ra'phanus with 

 the fundaments of the first 

 radial tubes, r. Po, inhalent 

 pores. 



