Introduction to Animal Morphology. 271 



IMyaceae — siphons long, united ; gills not extended thereinto ; 

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

 siphons covered with periostracum. Families : — Glaucono- 

 myidse — mantle lobes open sufficiently for the protrusion of 

 the keeled small foot. Solenidae — body long ; mantle open 

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

 cavidse — elongate foot with byssus groove ; cartilage small 

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

 (Panopaea). Anatinid^e — 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, &c.). Corbulidas — mantle lobes closed ; cartilage in a 

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

 their openings ; foot with a byssus groove. Myidse — 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 : Gastrochaenidas — 

 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- 

 pergillum). Pholadidae — body symmetrical ; mouth-lobes 

 long ; foot short ; hinge teeth none ; siphons either bearing 

 loose shelly plates at their end {the palettes of Teredo, the ship- 

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

 the stone-boring Pholas. Kuphus has palettes, but no true 

 shelly valve. Pholas has two triangular organs, covered with 

 cilia, and containing fat and granular protoplasm, near the 

 upper end of the mantle, which are the seats of its lumino- 

 sity {Panceri). 



