906 



POLYZOA AND TUN1CATA 



and some other Polyzoa a muscular stomach or gizzard for the tri- 

 turation of the food intervenes between the oesophagus and the 

 true digestive stomach.) The walls of the stomach, A, have consider- 

 able thickness, and the epithelial cells which line them seem to have 

 the character of a rudimentary digestive gland. This, however, is 

 more obvious in some other members of the group. The stomach is 

 lined, especially at its upper part, with vibratile cilia, as seen at c, g ; 

 and by the action of these the food is kept in a state of constant 

 agitation during the digestive process. From the upper part of the 

 stomach, which is (as it were) doubled upon itself, the intestine (i) 

 opens, by a pyloric orifice, /, which is furnished with a regular valve ; 

 within the intestine are seen at k particles of excrementitious matter 

 which are discharged by the anal orifice at I. No special circulating 

 apparatus here exists ; but the liquid which fills the cavity that sur- 

 rounds the viscera con- 

 tains the nutritive 

 matter which has been 

 prepared by the diges- 

 tive operation. and 

 which has transuded 

 through the walls of 

 the alimentary canal ; 

 a few corpuscles of ir- 

 regular size are seen to 

 float in it. No other 

 respiratory organs exist 

 than the tentacula, into 

 whose cavity the nutri- 

 tive fluid is probably 

 sent from the peri- 

 visceral cavity for aera- 

 tion by the current of 

 water that is continu- 

 FIG. 688. Cells of Polyzoa: A, Mastigophora Hynd- a ]i v flowi^or nvpr thpm 

 manniiB, Cribrilina figularis; cf Umbonula ^ * owm g over m< 

 verrucosa. The production of 



gemmce or buds may 



take place either from the bodies of the polypides themselves, which 

 is what always happens when the cells are in mutual apposition, 

 or from the connecting stem or ' stolon,' where the cells are distinct 

 one from the other, as in Laguncula. In the latter case there is 

 first seen a bud-like protuberance of the horny external integu- 

 ment, into which the soft membranous lining prolongs itself ; the 

 cavity thus formed, however, is not to become (as in Hydra and its 

 allies) the stomach of the new zooid, but it constitutes the chamber 

 surrounding the digestive viscera, which organs have their origin 

 in a thickening of the lining membrane that projects from one side 

 of the cavity into its interior, and gradually shapes itself into the 

 alimentary canal with its tentacular appendages. Of the produc- 

 tion of gemmae from the polypides themselves the best examples are 

 furnished by the Flustrce and their allies. From a single cell of the 

 Flustrse five such buds may be sent off, which develop themselves 



