Genera of the ISTorth American Palaeozoic Bryozoa. 453 



ingly thin transparent membrane, a prolongation of the outer 

 margin of the lophophore, fig. 22, 5, which is known as the calyx. 

 The calyx never exists in forms destitute of an epistome. 



Perigastric Space. 



The perigastric space is filled with a colorless fluid, which prob- 

 ably fulfils the triple purpose of circulation, nutrition and respira- 

 tion ; the last-named office probably in conjunction with the ten- 

 tacles, but there is no absolute certainty on this point. The fluid is 

 colorless and the circulation, which undoubtedly takes place, is de- 

 tected only through the medium of certain small particles, held in 

 suspension in the fluid ; but these particles are not necessary for 



Fig. 23. Tentacle enlarged, showing outer cellular and inner membranous layer, 



the performance of the functions of the fluid, because they are fre- 

 quently absent, and Mr. Hancock thinks that when they are 

 present, it is from some rupture of the walls. Dr. Farre has 

 described and figured parasites occurring in this fluid, but judg- 

 ing from his figure, he mistook the spermatozoa for parasites, 

 and he describes them as moving with a slow, undulating motion, 

 which is characteristic of the spermatozoa. 



Allman is of the opinion that the perigastric fluid is chiefly 

 aqueous and supplied from without In that case there must exist 

 pores through the ectocyst capable of being opened and closed 

 at the will of the animal or the water must exude through the 

 alimentary canal. What the other constituents of this fluid may 

 be has not yet been ascertained. Allman says : "That the fluid 

 thus contained in the perigastric space and thence admitted into 



