NUTRITION AND FEEDING MECHANISMS 221 



particles basally towards the mouth. Tentacles arising from the arms are 

 ciliated as well, and exhibit active jerking movements which may be 

 concerned with procuring and selecting food particles (31). 



Ascidians are provided with a branchial sac perforated by many small 

 apertures. By the beating of cilia on the sides of the gill-bars, sea water is 

 drawn into the branchial siphon and passes through the fine meshwork of 

 the pharyngeal basket on which fine particles are caught. These food 

 particles are collected by cilia on the pharyngeal surface of the gill-bars 

 and on their papillae, and are carried across the branchial surface towards 

 the dorsal lamina. In simple ascidians this process is aided by transverse 

 waving of the longitudinal bars. The filtering process is assisted by the 

 production of mucus-sheets, which are secreted by the endostyle and 

 transferred to the inner surface of the pharynx by endostylar cilia. On 

 reaching the dorsal lamina the food-laden mucous masses are carried 

 towards the posterior end of the branchial cavity and thence to the 

 oesophagus. 



Ascidians feed on plankton and detritus. On exposed shores the food 

 consists almost entirely of plankton, often enriched by a considerable 

 quantity of gametes and algal spores. But in protected waters and estuaries 

 it is made up largely of suspended matter and detritus, together with some 

 unicellular algae. Ciona intestinalis is capable of filtering off particles down 

 to 1-2// in size, and this is in large part due to the fine porosity of the 

 mucus-sheets produced, since when mucus secretion is in abeyance, small 

 particles of this size are no longer retained. 



In solitary ascidians ciliary activity is augmented by rhythmical squirting, 

 caused by quick contractions of the body wall. The steady ciliary current 

 of Phallusia mammillata amounts to 60 c.c. per hour, while spontaneous 

 squirting moves some 300 c.c. of sea water per hour in a fasting animal. 

 In a hungry animal, rhythmic squirting increases greatly in frequency, 

 whereas the addition of food restores the frequency to normal (Fig. 5.17). 

 Squirting renews the sea water in branchial and atrial chambers, brings a 

 fresh lot of food particles into the branchial sac and periodically renews the 

 water about the animal (47, 52, 64). 



In thaliacians (Salpa, Doliolum) the endostyle secretes a mucous net of 

 entangling threads into the pharyngeal cavity. Food particles caught in the 

 mucus are transported to the dorsal pharynx by peripharyngeal cilia, and 

 thence to the oesophagus by other ciliary tracts. In doliolids the through- 

 current is produced by cilia of the branchial stigmata ; in salps, by rhythmic 

 contractions of circular muscles of the body wall (13, 52). 



The small appendicularians (Larvacea) have a most remarkable method 

 of feeding. These animals are pelagic in habit and dwell in a gelatinous 

 house which is secreted by the animal and forsaken from time to time, when 

 a new one is formed (Fig. 5.18). The house is essentially a filtering appara- 

 tus for straining off fine particles and nannoplankton on which the animal 

 depends for food. Water currents, created by lashing of the animal's 

 tail, enter through a pair of dorsal funnels provided with a fine grating for 



