Introduction to A nimal Morphology. 271 



Myaceae siphons long, united ; gills not extended thereinto ; 

 shell gaping behind, with an inner hinge cartilage ; shell and 

 siphons covered with periostracum. Families : Glaucono- 

 myidae mantle lobes open sufficiently for the protrusion of 

 the keeled small foot. Solenidce body long ; mantle open 

 fore and aft ; hinge anterior, terminal ; byssus none. Saxi- 

 cavidae elongate foot with byssus groove ; cartilage small 

 or none ; pallial line interrupted (Saxicava), or continuous 

 (Panopaea). Anatinidae quadriforal ; hinge with ossicles ; gills 

 unilamellar, the one larger in Thracia being subdivided into two 

 half lamellae ; shell with an outer layer of fine prismatic cells 

 (Pandora, Mydora). Mactridae ligament internal ; siphons 

 united ; labial tentacles long, pointed ; foot straight ; shell 

 gaping (Lutraria) or closed ; outer ligament separated from 

 the cartilage by a shell ridge (Trigonella, &c.), or united to 

 it (Rangia, &C.) ; mantle lobes separate (Mactra, &c.), or united 

 (Lutraria, ^Vc.). Corbulidx mantle lobes closed ; cartilage in a 

 spoon-like pit ; oral palp small ; siphons short, with cirri at 

 their openings ; foot with a byssus groove. Myidae mantle and 

 cartilage as in last ; foot not byssiferous. Sub-order 9. Phola- 

 daceae siphons long, united; gills elongated into them; shell 

 small, only covering part of the body, whose surface often se- 

 cretes a calcareous tube ; borers. Families : Gastrochaenidae 

 tube-dwellers ; body clavate ; shells small, equivalve ; tube closed 

 in front ; shell valves free in the tube (Gastrochaena), or the left 

 attached and the right free (Clavagella), or both fused (As- 

 im i ). riinl.xlulx body symmetrical ; mouth-lobes 

 foot short ; hinge teeth none ; siphons either bearing 

 loose shelly plates at their end (the //*//* of Teredo, the ship- 

 worm), or with no palettes and rough, sculptured shells, as in 

 . Kuphus IMS jialrth's, but no true 

 Pholas has two triangular organs, cove-red with 

 cilia, and containing fat and granular protoplasm, near the 

 :he mantle, which arc the beats of its lumino- 

 sity ( / 



