196 THE BIOLOGY OF THE SEA-SHORE 



from the effects of wave action. Communication with 

 the surface is maintained by means of siphons containing 

 inhalent and exhalent passages, through which the ciHa 

 of the gills maintain a constant current of water. Recent 

 research has shown that the chief function of the gills is to 

 act as a food-sieve, respiration being effected chiefly by the 

 mantle. According to Orton (1912), evolution in Lamelli- 

 branchs has mainly followed the lines of folding and consoli- 

 dating the gill-filaments in order to ensure a more perfect 

 method of feeding. 



The method of feeding of Ascidians is essentially 

 similar to that employed by other sedentary forms. That 

 is to say, it depends on causing a stream of water to circulate 

 through the body. In this case the circulation is effected 

 by the cilia on the margins of numerous perforations in the 

 anterior part of the gut (the pharynx or branchial sac). 

 The food particles, drawn in with the water current into 

 the branchial sac, become enveloped in mucus secreted by a 

 median ventral groove called the endostyle, and are trans- 

 ported by means of a circular groove at the anterior end of 

 the pharynx to a median dorsal " lamina." From here they 

 are driven backwards by ciliary action into the digestive 

 part of the gut. Herdman (1904) has estimated the number 

 of gill openings in an adult Ascidia mentula of medium size 

 to be about 192,000, and, as there is a row of cilia on each side 

 of every slit, there are in all about 384,000 short rows of 

 cilia lashing water through the pharynx of the animal. The 

 process of feeding is automatic, but there is evidence that 

 selection of food can, to some extent at least, be exercised. 



Young fish, a number of which frequent tidal waters, 

 feed in various ways . Some deliberately stalk certain selected 

 food, others apparently eat the first thing that comes along ; 

 some feed at all times, others only by night ; some eat very 

 little at one time, others as much as they can get. Some, 

 like the Ballan Wrasse, are clumsy ; others, like the whiting, 

 are very quick in their movements. Young lumpsuckers 

 are very active and unfix their suckers instantly to chase 

 and swallow anything attractive. The rockling {Onus 



