EXTANT GASTK^ADS. 



67 



of Haliphysenm forms a most simple, cylindrical or egg- 

 shaped pouch, the wall of which consists of two cell-strata. 

 The cavity of the pouch is the stomach-cavity, and the 



Figs. 180, 181. — Haliphysema prlmordiale, an extant Gastrasa-form. 

 Fig. 180. External view of the whole spindle-shaped animal (attached by 

 its foot to seaweed). Fig. 181. Longitudinal section of the same. The 

 primitive intestine (d) opens at its upper end in the primitive mouth (m) . 

 Between the whip.oells (g) lie amoeboid eggs (e). The skin-layer (h) below 

 is encrusted with grains of sand, above with sponge-spicules. 



opening at the top is the mouth-opening (Fig. 181, m). 

 The two cell-strata forming the wall of the pouch are the 



