206 



THE LAMELLIBRANCHIA 



is only furnished with a pair of labial palps on each side. The foot 

 is ventral, generally adapted to burrowing and without a plantar 

 surface. The two pallial lobes are united by one or two transverse 

 muscles which close the two valves of the shell. There are two 

 lateral and symmetrical ctenidial branchiae under the mantle ; their 

 distal extremities are directed posteriorly and their filaments may 

 exhibit an extreme degree of concrescence, either among themselves 

 or with the mantle or with the visceral mass. 



I. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. 



The mantle consists of two thin tegumentary lobes attached 

 to the trunk dorsally, and extending over the sides to the ventral 

 surface so far that they can be brought together below the foot. 

 Thus the whole of the animal's body may be covered by the mantle. 

 In structure, the mantle is normally rather thin, and there is only 

 some connective tissue and a few muscular fibres between the internal 

 and external layers of epithelium. In some few cases the gonads 

 may extend into the mantle : into both lobes in the Mytilidae, into 

 the right lobe only in the Anomiidae. The internal surface may 

 present glandular modifications, of which the most important are the 



hypobranchial glands characteristic of 

 the Protobranchia : they are situated pos- 

 teriorly beyond the gills. The margins 

 of the mantle normally present redupli- 

 cations, generally three in number 

 (Fig. 187, pa, pa", pa'"}, and in the 

 Pectinidae the most internal of the three 

 is turned inwards to form the "velum" 

 (Fig. 235). The margins of the mantle 

 of Lamellibranchs are frequently fur- 

 nished with glands, pigment spots, and 

 various sensory organs in the form of 

 tentacles, and even of eyes. The pig- 

 mentation of the pallial border is due 

 to the combined action of the light and 

 the oxygen of the surrounding medium, 

 Montagu, and is most conspicuous at those points 

 -terior adductor- o' ' L inhalant w ^ ere the respiratory fluid enters the 



orifice; o", pedal orifice; o"', ex- pallial Cavity. 



halant oritice ; pa', external duplica- m-, , ,. , ,,. , . . 



The edges of the two pallial lobes 

 may remain free throughout their 

 extent; this is the case in Nucula, 

 the Anomiidae, the Arcidae (Fig. 188) 



the Trigoniidae, and the Pectinidae (Fig. 235). In all other 

 Lamellibranchia the pallial lobes are partially united by the 



cud 



FIO. isr. 

 Keiiya suborbimiaris, 



ture of the pallial edge ; pa", internal 

 ditto; pa"', median ditto; su', su", 

 first and second sutures of the pallial 



edges. (After Deshayes.) 



