in. 4 



ORGANIZATION OF CIONA 



61 



Ascidians are often brightly coloured, the pigment being either in the 

 tunic or the underlying body, which shows through the transparent 

 tunic. The colour can change, at least over a period of some days. Little 

 is known about the origin of the pigment, but it is sometimes derived 

 from the blood-pigment and may lie in pigment cells. 



The mantle is provided with muscle-fibres running in various 

 directions but mainly longitudinally, and serving to draw the animal 

 together, with the production of 

 the jet of water from which the 

 animals derive their common 

 English name. 



The greater part of the body is 

 made up of an immense pharynx, 

 beginning below the mouth and 

 forming a sac reaching nearly to 

 the base (Fig. 32). The sac is 

 attached to the mantle along one 

 side (ventral) and is surrounded 

 dorsally and laterally by a cavity — 

 the atrium. This pharynx is, of 

 course, the food-collecting appa- 

 ratus ; its walls are pierced by rows 

 of stigmata (gill-slits) whose cilia 

 set up a food current entering at 

 the mouth and leaving from the 

 atriopore. The entrance to the 

 pharynx is guarded by a ring of 

 tentacles, which may be compared 



with the velum of amphioxus. The stigmata are very numerous 

 vertical cracks, all formed by sub-division of three original gill- 

 slits. Tongue bars grow down to divide each slit and then from each 

 tongue bar grow horizontal synapticulae. This arrangement has clear 

 resemblance to that of amphioxus and results in the production 

 of a pharyngeal wall pierced by numerous holes. Immediately within 

 the stigmata there is a series of papillae, provided with muscles and 

 cilia. There is an endostyle, which has three rows of mucus cells 

 on each side, separated by rows of ciliated cells and with a single 

 median set of cells with very long cilia (Fig. 33). The mucus secreted 

 in the endostyle is caught up on the papillae, whose muscles move 

 them rhythmically, spreading a curtain of mucus over the inside of 

 the pharynx. Food particles are caught in the mucus, which moves 



Fig. 33. Transverse section of the endo- 

 style of Ciona. 



lat. cil. lateral cilia; med. cil. long median cilia; 

 mu. mucous cell. (After Sokoloska.) 



