Mr. C. P. Sigerfoos on the Pholadidaj. 253 



'C5 



(Pe) are as much concentrated as they will become. The 

 otoliths lie lateral to their posterior edges and persist to the 

 adult condition. The visceral ganglia (V) are still wide 

 apart and lie in front of the posterior adductor muscle (Vi). 

 With their commissure thej form a cylindrical mass four or 

 five times as long as its diameter and somewhat enlarged at 

 the two ends, the ganglia proper. The ganglion of the 

 respiratory nerve {Bg) is still wide apart from the visceral. 



The alimentary canal is already highly specialized. A 

 rather long oesophagus (0) leads into the stomach (S), on 

 either side of which a single liver-lobule is borne. From 

 the 'posterior part of the stomach the sheath of the crystalline 

 style (*S»S) has grown, and, though small, its wall is already 

 composed of large, clear, slightly granular cells, bearing long 

 dense cilia. The intestine leaves the right side of the 

 stomach in front, forms a single loop, and bends upwards and 

 backwards to hang over the posterior adductor [Re). From 

 the middle of the right side of the stomach the csecum has 

 grown as a small hemispherical bag, though it is still 

 functionless. 



The renal organs are present and lie ventral to and in front 

 of the posterior adductor muscle. Near tiie cerebral ganglion 

 on either side is a glandular structure which I shall describe 

 later. 



Metamorphosis and Adult Structure. 



Once attached, the larva probably never leaves its place. 

 The velum is very soon lost, and has an interesting fate. 

 The long lower lip is thrown forward to the mantle in front 

 under the velar cavity, so that the cells of the velum which 

 are cast oj^'have but one mode of escape, into the oesophagus. 

 They break off from the basement membrane and are eaten. 

 Within a ^tvf hours the membrane contracts so as to wholly 

 obliterate the velar cavity. Tiiis fate of the velum is inter- 

 esting, for it has been described as entering into the formation 

 of the labial palps. But in this species there are no palps, 

 and it would not do to generalize to other forms. However, 

 in the oyster, where the palps become very large, they are 

 very small at first, much smaller than the velum, though 

 they are not developed till after the oyster has become 

 attached. 



The foot very soon (within a day) becomes the pestle- 

 shaped cupping organ, which assists the shell in boring. 

 The byssus apparatus is functional for a day or two, and then 

 degenerates, though it persists in the posterior part of the 

 foot as a small closed vesicle, as in Gyclas, 



