Gexeea of the E'oeth American Palaeozoic Bryozoa. 451 



In the Hippocrepian forms, one side of the lophophore, that 

 on which the anus is situated, is prolonged in two triangular 

 arms. The tentacles are arranged continuously on the outer and 

 inner margins, making a Rouble row (fig. 23), in the form of a 

 horseshoe, a fact from which the name Hippocrepian is given to 

 this class of Bryozoa. This condition is not found in any marine 



form. 



The Hippocrepian forms have also another organ which is 

 absent in those genera in which the lophophore forms a continu- 

 ous circle. This is a hollow valve-like organ continuous with one 

 side of the lophophore, and is known as the epistome. The in- 

 terior communicates with the perigastric space by an opening in 

 the lophophore (fig. 22, k). The walls which are turned toward 

 the mouth are thick and are covered with a vibratile cilia ; the 

 others are membranous and transparent. 



Fig. 21. Membranipora pilosa. In one of the cells the animal Is partially retracted, in the other 

 it is fully expanded (after Farre) . 



When the animal is protruded, this organ is in constant mo- 

 tion, elevated and depressed. The elevation is effected by the 

 action of a muscle (fig. 22, Z), to be hereafter described ; while the 

 depression is probably effected by the antagonistic elasticity of 

 the organ itself, though it may be effected by the action of a 

 muscle not yet observed. 



Notwithstanding this organ has been studied by many observ- 

 ers, its purpose has not yet been ascertained. 



