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

 at tachment proceeds with uncommon rapidity. 

 ^^bn the gastrula-stage ectoderm and entoderm are not 

 ^^rosely 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. According 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 

 has 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 (o-fculum, 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. 6a, 

 Olynthus). 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. 5b. — Older, attached 

 stage of Sycandra raphanus with 

 the fundaments of the first 

 radial tubes, r. Po, inhalent 

 pores. 



