NUTRITION AND FEEDING MECHANISMS 



205 



In particle feeders the cytopharynx is approached by a spiral groove, 

 the peristome, which leads from the anterior end to the pharyngeal opening 

 or cytostome. The peristome, like the pharynx, is ciliated. The ciliary pattern 

 is fairly simple in holotrichs (Paramecium, Colpoda); but in higher forms 

 the cilia on the outer edge of the peristome are fused so as to form a row 

 of cirri or membranelles, the adoral wreath, which produces a powerful 

 current. A few characteristic marine forms from different orders may be 

 mentioned. Bottle animalcules Folliculina (Heterotricha) have the peris- 

 tomial area enlarged into paired wing-like extensions. In planktonic 

 tintinnids (Oligotricha) the peristome bearing the adoral wreath and mouth 

 opening occupies the anterior end of the body. In sessile peritrichs such as 

 Vorticella the peristome forms a disc at the anterior end. The peristome 

 bears a spiral groove, which is provided with cilia in the form of an 

 undulating membrane and which descends to the 

 vestibule (chamber leading into the cytopharynx in 

 these forms). 



Free-living ciliates ingest bacteria, algal cells, 

 diatoms and other protozoa, and probably a certain 

 amount of detrital matter. Littoral sand-dwelling 

 holotrichs appear to derive nourishment from 

 diatoms. In peritrichs the formation and course of 

 food vacuoles have been described in some detail. 

 Food vacuoles, containing bacteria, are constricted 

 off from an oesophageal sac at the bottom of the 

 cytopharynx. This vacuole then passes rapidly on a 

 fixed course through the cytoplasm, while its contents 

 are undergoing digestion, and returns to an anal spot 

 on the wall of the vestibule for discharge (48, 71). 



Porifera. Among sponges the feeding water cur- 

 rent is created by the lashing of flagella on collar 

 cells or choanocytes which line internal cavities 

 (Figs. 5.1, 5.2). The surface of the sponge is perforated by dermal pores 

 like a sieve, and water drawn through these openings flows past the collar 

 cells. The choanocytes beat in an independent and unco-ordinated 

 manner, and as the water currents sweep past the cell bodies, suspended 

 food particles are caught and ingested in an area near the base of the 

 collar. 



The simplest type of sponge structure, the ascanoid type, is realized in 

 Olynthus, a fleeting stage in the development of calcareous sponges. It is 

 a hollow vase-shaped structure in which the internal paragastral cavity is 

 lined with choanocytes. The pores in the body wall are surrounded by 

 porocytes, capable of constriction; the osculum or terminal exhalant 

 opening of the body cavity can also be closed by the action of myocytes. 

 The ascanoid type is found modified to some extent in the calcareous 

 sponges Clathrina and Leucosolem'a, which are colonies made up of branch- 

 ing ascon tubes. In the syconoid type, found in Sycon and Grantia (Cal- 



Fig. 5.1. Collar 

 Cells of a Sponge, 

 Leucosolenia cori- 

 acea. (From Min- 

 chin, 1909.) 



