76 



CASWELL GRAVE 



MANTLE 



The mantle (ectoderm) varies in thickness in different re- 

 gions of the body, but it consists at no point of more than a single 

 layer of cells. The cells of the mantle in general are more or 

 less cubical in form, but are high and columnar in the parts 

 forming the rudiments of the oral and atrial siphons, thin and 

 pavement-like in the region above the sensory vesicle and in 

 the mantle sheath of the tail (figs. 1, C, 3 and 8). 



tu'es. 



t.yes. 



P^Ht 



t. ves. int. 



Fig. B A drawing of the embryo within its chorionic membrane, showing the 

 twisted and coiled tail and the four points along the median keel of the body 

 at which the test-clubs grow out from the mantle to form the test vesicles. 



ADHESIVE PAPILLAE 



The adhesive papillae, of which there are three arranged in a 

 vertical series at the anterior end of the body, are tubular out- 

 growths of the body wall (fig. 1). Each terminates at the sur- 

 face of the tunic in an enlarged, goblet-shaped body which opens 

 outwardly and contains a large lens-shaped mass of elongated, 

 richly granular cells probably of mesenchyme origin. The cen- 

 tral canal of each papilla is partially filled with mesenchyme 

 cells (fig. 7). Toward the end of the free-swimming period 

 of the tadpole, a contraction of the wall of each papilla takes 

 place, causing the contents of the terminal, cup-shaped enlarge- 

 ment to be extruded upon the surface of the tunic. The viscid 



