OMNIVOROUS BIRDS AND PROTOCHORDATES 



245 



ciliary action along two grooves, one running along the upper side 

 of the pharynx and the other along the lower. The Lancelet 

 feeds with its body buried vertically in the sand, its front end 

 projecting (fig. 461). 



SEA-SQUIRTS (Urochorda) 



Lower in the scale than the Lance- 

 let are the remarkable and degener- 

 ate Sea - Squirts or Ascidians (see 

 vol. i, pp. 297-300). These are either 

 solitary or aggregated into colonies, 

 and may be either fixed to stones, &c., 

 or may float freely in the sea. It will 

 be sufficient to describe briefly the 

 feeding arrangements in a simple 

 fixed form, such as Ascidia (see vol. i, 

 p. 297). We find here a sac-like 

 body, covered by a firm protective 

 coat or test, and fixed by one end. 

 At the other end is the mouth, and 

 close to it another aperture from which 

 currents flow out carrying the various 

 products of waste. The mouth leads 

 into a small mouth -cavity, and this 

 again into a very large pharynx, the walls of which are per- 

 forated by a great number of small holes (gill-clefts). The 

 pharynx is suspended in an atrial cavity, comparable to that 

 of the Lancelet, and as in that animal opening to the exterior, 

 though in Ascidia the aperture, as already mentioned, is quite 

 close to the mouth. Feeding is effected much as in the 

 Lancelet, i.e. currents of sea- water set up by the numerous cilia 

 which beset the lining of the pharynx enter the mouth, bearing 

 with them all sorts of food particles. These are entangled in 

 mucus and carried back to the mouth, while the water passes 

 through the gill-slits, purifying the blood as it does so, and reaches 

 the atrial cavity, thence sweeping out waste products to the 

 exterior. If the animal be touched it shrinks up, ejecting the 

 sea-water contained in the pharynx with some force, and closing 

 the external apertures. This is no doubt a protective measure, 

 and it should also be noted that the beginning of the pharynx is 



Fig, 461. — Lancelet {Amphioxus 

 lanceolattis]y feeding 



