346 MOLLUSCA. 



Sub-order 3. MYTILACEA. 



Symmetrical; mantle-lobes fused posteriorly; anterior adductor smaller than 

 posterior; a single aorta; gill-filaments with interlamellar junctions; genital 

 glands extending into the mantle, and opening by the side of the kidney 

 openings. 



Fam. 4. Mytilidae. Mussels. Attached by the byssus fibres of the tongue- 

 shaped foot. Mytilus L. ; M. edulis L. (Fig. 256), edible mussel of North Sea 

 and Baltic ; Modiolaria Loven ; Modiola Lam. ; Lithodomus Cuv. ; L. dadylus 

 Sow., Mediterranean (Temple of Serapis at Pozzuoli). 



- 

 Order 3. PSEUDOLAMELLIBRANCHIATA. 



Mantle entirely open; gills folded and filament at reentrant angle of fold 

 modified; inter filamentar junctions effected by ciliated: discs or vascular con- 

 crescence. 



Foot feebly developed ; anterior adductor usually absent ; the auricles 

 communicate with one another ; the branchial filaments have interlamellar 

 concrescences ; genital glands open into or near the kidneys. 



Fam. 1. Aviculidae. Byssus -apparatus well developed ; shell usually un- 

 equivalve, dorsal margin straight and often long ; ascending limbs of outer 

 gill-filaments fused to mantle. Avicula Klein (Fig. 252) ; Pinna L. ; Perna 

 Bruguiere; Meleagrina Lam. ; M. margaritifera L., pearl-oysters, Indian Ocean, 

 Persian Gulf, and Gulf of Mexico ; Malleus Lam. : Vulsclla Lam. 



Fam. 2. Ostreidae. Byssus absent ; fixed by shell ; outer gill-filaments fused 

 to mantle ; shell unequi valve, fixed by left valve. Ostrea L. Oysters. Shell valves 

 unequal, laminated, with weak hinge usually without teeth, single somewhat 

 ventrally placed adductor muscle. In the true oysters the more arched left 

 valve is firmly attached, while the right and upper valve, which is fastened 

 by an internal ligament, lies as an operculum on the lower valve. Edges of 

 mantle fringed, not fused ; gill lamellae fused with mantle and across the 

 middle line. Foot absent or rudimentary. They usually live together, and 

 form banks of considerable extent. Found in the Jura and the Chalk. 

 Ostrea edulis L., on the coasts of Europe on rocky ground ; probably 

 includes a number of different species according to the locality. According 

 to Davaine, the oysters are said to produce only male sexual products towards 

 the end of the first year, and it is not until later, from the third year 

 onwards, that they become females and produce ova. Moebius, on the con- 

 trary, asserts that the sperm is the later formed, and not until after the 

 pregnant beast has got rid of her eggs. The reproduction takes place especially 

 in the months of June and July, at which time in spite of their extraordinary 

 fertility the oysters should not be gathered. In the American oyster (0. 

 mrginiana] and the Portuguese oyster (0. angulata] the sexes are separate. 

 The green oyster owes its colour to its food a diatom Namcula ostrearia* 

 The colour is confined to its gills and labial palps, and is said to be due 

 to selective absorption of the pigment from the blood. It is, however, possible 

 that the diatoms may adhere to the surface of the gills and palps, and be 

 consumed by phagocytes or other cells. This suggestion requires testing. 



Fam. 3. Pectinidae. Byssus absent or feebly developed; shell ribbed; 

 mantle-edge tentaculiferous ; a duplication of mantle-edge folded internally ; 



* Ray Lankester, Q. J. M. S., 26, p. 71. 



