248 LECTURES ON MOLLUSCA. 



rows in floating wood, against the grain, about an inch long. The 

 body is globular, with narrow pipes, separated at the end. 



In the " Cup-pholas" tribe, the foot opening is large in the young 

 shell, but closed in by shelly matter in the adult. There are however 

 transition forms.. Plioladidea has a single large cup in the adult, but 

 no accessory plates. In the African Talona, there are two small cross- 

 plates; and the foot-gape is very small, both in the young and adult. 

 Martesia burrows in floating wood, and has the valves divided into 

 two areas, like Plioladidea ; it differs in having a large shield over the 

 beaks, with another along the back; and in having no cup. One 

 species has been found living in a Borneo river, twelve miles from the 

 sea. The curious west American genus Parapholas has the valves 

 divided into three areas, the third consisting of a tiled row of cup- 

 plates. The adult is encased in large accessory plates, in front as 

 well as behind. In this group the foot-gape in the adolescent aniihal 

 is guarded by a strong deposit of shelly matter, to prop up and aid 

 the foot. Jouannetia is like an exaggerated Parapholas, in which the 

 callous plate of one valve overlaps the other, and the tile-cups are 

 almost obsolete. As in the other members of this section, the pipe- 

 ends are joined and surrounded by a common fringe, accounting for 

 the roundness of the burrow-mouths. The Cup-pholads are found 

 fossil in the secondary rocks ; the ordinary forms in the tertiary strata. 



Family Teredld;e. {Ship- Worms.) 



TheShip-wormsare simply Pholads enormously lengthened ; although 

 at first sight the shape of their body would cause them to be regarded 

 as Annelids or Vermetids, rather than bivalve mollusks. The com- 

 mon Teredo has a body from one to two and a half feet long; i. e. in- 

 cluding the pipes ; but the body, strictly so called, which contains the 

 principal viscera, and is enclosed in a bivalve shell, open at each end 

 like a pair of pincers, is not larger than a pea. The gills are long 

 and extend into the tube, which is protected by a coat of shell outside. 

 At the outer end, where the pipes divide, there are a pair of shelly 

 flaps, which aid in working the inhalent and excurrent siphons. 

 These flaps, which look like the " screw"-plates of a steamer, might 

 be mistaken for the boring apparatus, but that they are always found 

 at the opposite end from the boring foot. This is finger-shaped, as in 

 Gasliochcena ; but it is quite equal to the task of wood-boring. There 

 is no mollusk except the Ship-worm, which has excited the fears of 

 merchants and statesmen. Not only ships, (if not coated with metal 

 or kyanized,) but piles and dock gates, have fallen victims to its 

 ravages. Nevertheless it is a very serviceable creature, gradually 

 destroying wrecks and other submerged wood, which might otherwise 

 block up harbors and impede navigation. They are ovoviviparous 

 and very prolific. They always bore with the grain, only turning 

 aside to avoid knots or neighborly intrusion. In Xylotrya the breath- 

 ing flaps are pen-shaped and jointed. Some of the species are found 

 boring in the floating husks of cocoanuts. 



There is a curious group of Sand-worms, as yet very little under- 

 stood, but closely related to the Ship-worms. They encase themselves 



