274 PROCEEDINGS OF THE ACADEMY OF [1889. 



NOTES ON THE ANATOMY OF PHOLAS (BARNEA) COSTATA LINNE, 

 AND ZIRPHiEA CRISPATA LINNE. 



BY W. H. BALL. 



In P. costata the mantle is entirely closed, except for the passage 

 of the foot. The siphons of the specimen (contracted in alcohol) 

 combined in a single envelope with distinct terminal orifices, are 

 little shorter than the whole of the rest of the body; their surface is 

 finely circularly wrinkled, they have no epidermidal coat and no 

 terminal coriaceous appendages. The papilla 3 around the two orifices 

 are small and inconspicuous. The mantle margin is simple; the 

 median line of the connective tissue joining its edges is marked off 

 by a pair of not very prominent raphes. The aperture for the foot 

 is oval, about one quarter as long as the shell. About it is a smooth, 

 thick membrane extending laterally to a raised papillose ridge, the 

 anterior prolongation of either raphe, which separates it by a narrow 

 space from the much thinner simple margin of the mantle, which is 

 continuous with the epidermis. The tissue of which this encircling 

 membrane is composed is thick ; within the aperture, extending a 

 little behind it, on each side is a sort of curtain whose office 

 apparently might be to close around the foot and prevent the influx 

 of sand or mud. The anterior ends of these curtains do not reach 

 as far forward (by a distance about equal to a third of their whole 

 length) as the anterior commissure of the pedal opening. On open- 

 ing the mantle-cavity we are first struck by the immense size of the 

 labial palps; the anterior or external palpus is adnate throughout 

 the greater part of its extent upon the inner surface of the mantle; 

 only a small anterior border and its lateral tips being free. It is 

 radiately striate, with transverse close-set lamellae on its free mar- 

 gin. The lower or posterior palpus is very thick and cellular, with 

 a lamellar gill-like surface internally, but smooth on its outer face. 

 It is produced laterally into long slender points which extend back- 

 ward further than the pedal opening. It is not muscular, at least 

 to any great extent, and is supported by the apophyses proceeding 

 from under the beaks of the shell ; these processes are buried in its 

 substance, though their distal margins also penetrate the visceral 

 mass internally for a short distance beyond the palpi. The foot 

 may be said to form the ventral face of the whole visceral mass ; it is 

 flattened, laterally carinated and terminates behind in an acute free 



