vii MOLLUSC A INTEGUMENT, MANTLE, VISCERAL DOME 51 



(b) In most Eulamellibranchia, viz. in most Lucinidce, most Cyrenidce ; among 

 the Unionidce, in the Mutclince, in the Donacidce, Psammobiidce, Tellinidce, Scrobi- 

 culariidce ; among the Veneracea, in the Veneridce, in the Cardiidce, the Mactridce, 

 Mesodesmatidce, and the Solcnidce (excepting Solen and Lutraria). 



(c) In all Septibranchia (Poromyia, Cuspidaria). 



In the above forms the mantle is still wide open, i.e. the points of concrescence 

 are small and local. But these points may become lines of concrescence of con- 

 siderable length. In the Chaniacea, for example, and especially in the Tridacnidce 

 among the Eulamellibranchia, the three apertures of the mantle are found at con- 

 siderable distances from one another, being divided by long intervals where the 

 edges have grown together. 



In some groups of Lamellibranchia, the concrescence between the anal and 

 branchial apertures or siphons remains short, i.e. the one aperture lies directly 

 below the other, but in such cases the edges anterior to the branchial aperture 

 grow together to a greater extent, so that the pedal aperture becomes reduced to 

 a small anterior fissure. In this condition the mantle is closed. Such a mantle is 

 found : 



Among the Eulamellibranchia in the Modiolarca, Dreissensia, Petricola, all 

 Pholadidce (Pholas, Pholadidea, Jouannetia, in which the pedal aperture is said 

 to close entirely in old animals, Xylophaga, Martesia) ; in the Teredinidce, and 

 among the Pandoridce, Pandora, the Verticordiidce and Lyonsiidce (Anatinacea). 



D. There are some Lamellibranchia with closed mantle, in which a fourth 

 aperture is added to the three found in the above groups, the mantle thus having 

 three points of concrescence. The fourth aperture is always small, and is found 

 between the pedal and branchial apertures ; it probably corresponds with a rudi- 

 mentary fissure for the byssus. 



This arrangement is found in the Eulamellibranchia; among the Solcnidce, in 

 Solen and Lutraria ; among the Pandoridce, in Myochama ; in Glycymeris ; among 

 the Anatinacea, in the genus Thracia ; in the Pholadomyidce and the Clavagellidcc 

 (Clavagella and Brcchites [AspergillumJ) ; and, finally, in Lyonsia norvegica. 



The anal aperture is often and the branchial aperture nearly always 

 fringed, or in various ways edged with protuberances, papillae, or ten- 

 tacles, and this is the case whether these apertures are found on the 

 edge of the mantle or at the ends of (longer or shorter) siphons. 



The siphons can be contracted and extended, and either wholly 

 or partly withdrawn into the shell, by means of special muscles. 

 These muscles are attached on the inner surface of the shell-valves to 

 the right and left posteriorly, and their line of attachment forms the 

 pallial sinus, which will be described later on. 



The length of the siphons varies greatly. Specially long siphons are found in 

 the Mactridce, Donacidce, Psammobiidce, Tellinidce, Scrobiculariidce, many Veneracea 

 and Cardiidce, the Mesodesmatidce, Lutraria, the Pholadidce, Teredinidce, Anatinidce, 

 and Clavagellidce. 



The siphons may be separated throughout their whole length, and often diverge 

 one from the other (e.g. Galatea among the Cyrenidce, the Donacidce, Psammobiidce, 

 Tellinidce, Scrobicularidce (Fig. 58), Mesodesmatidce, Pharus, etc.). 



In other forms they coalesce along their entire length ; they may even look like 

 a single tube, which is, however, always internally divided into an upper (anal) and 

 a lower (branchial) channel. This common siphon is sometimes protected by 

 a special sheath of epidermis, particularly in those forms in which it cannot be 



