ANATOMY OF A FRESH-WATER MUSSEL. 51 



4. The ascending lamella of the inner gill is concrescent 

 anteriorily with the foot. 



5. The ascending lamellae of the inner gills are concrescent 

 with each other posteriorily. 



6. The union of the gill-axis with the body-wall may be partly 

 due to concrescence." 



The surfaces of the gill -filaments are covered with cilia, which 

 may be divided into three group*, viz., those with the shortest 

 cilia (lateral], those with medium-sized cilia (frontal}, and those 

 consisting of a single row of cells which have very long cilia 

 (lateral frontal). The lashing of these cilia produces a current 01 

 water, which passes in the infra-branchial chamber by the inhalent 

 aperture, and, after finding its way into the supra-branchial chamber 

 through the fenestrse in the gill-lamellae and the slit between the 

 visceral mass and the inner gill, leaves the body through the 

 exhalent aperture. Professor Ray Lankester thinks that perhaps 

 the gills and labial palps are the homologues of the prae-and post- 

 oral ciliated bands of the Echinoid and Ophiurid larva, Pluteiis, 

 or the Tornaria larva of Balaiwglossus. Doubtless, as Peck 

 thinks, the current of water produced by these cilia subserves 

 the function of alimentation as well as that of respiration. 



THE DIGESTIVE SYSTEM. The mouth is situated below the 

 anterior adductor muscle, and is bounded by a pair of membranous 

 folds on each side the labial palps which lessen in size the 

 nearer they are to the mouth. The labial palps at the mouth are 

 confluent on either side, and their free edges encircle a groove, 

 known as the ciliated groove, which runs into the mouth-aperture. 

 I have just referred to what Professor Lankester has suggested 

 regarding these palps taken together with the ctenidia ; it would be 

 as well to mention that Loven has suggested that the palps are 

 the remains of the velum. The oesophagus is short but spacious, 

 and leads back into a dilated chamber of an irregular shape the 

 stomach which lies in the substance of the digestive-gland in a 

 position in the body immediately anterior to the pericardium. 

 The intestine commences on the floor of the stomach by a well- 

 marked pylorus, runs downwards and backwards towards the 

 posterior end of the body in intimate relation wiih the generative 

 gland, then doubling on itself, it passes again downwards and 



