Japan Current — See Kuroshio. 



jellyfish — (or medusa). 1. Any of various free- 

 swimming coelenterates having a disc- or bell- 

 shaped body of jellylike consistency. Many have 

 long tentacles with nematocysts (stinging 

 cells ) . Some are capable of producing glowing 

 ball luminescence. 



2. Any jellylike free-floating organism. The 

 term jellyfish often is applied to the cteno- 

 phores and may be to certain tunicates. 



jetsam — See jettison. 



jettison — The throwing overboard of objects, es- 

 pecially to lighten a craft in distress. Jettisoned 

 objects that float are termed flotsam ; those that 

 sink, jetsam ; and heavy articles that are buoyed 

 for future recovery, lagan. ^See derelict. (68) 



jetty — 1. In United States terminology a struc- 

 ture, such as a wharf or pier, so located as to in- 

 fluence current or protect the entrance to a har- 

 bor or river. A jetty extending into the sea to 

 protect the coast from erosion is called a groin. 

 A jetty which breaks the force of the sea at any 

 place is called a breakwater. A jetty, wall, or 

 bank, often submerged, built to direct or confine 

 the flow of a river or tidal current is called a 

 training wall. A wall or embankment along a 

 waterfront, to resist encroachments of the sea, 

 is called a sea wall. 



2. In British tenninology a pier, usually 

 of solid construction, intended as a berthing 

 place for ships. 



. (68) 



jingle shell — One of a family (Anomiidae) of 

 rounded bivalves with asymmetrical, thin al- 

 most transparent valves. The individuals are 

 attached to firm surfaces by a calcified byssus 

 projecting through a hole in the lower smaller 

 valve. These organisms are abundant foulere 

 in some regions. 



Jog-Log — A towed electrode that can detect ocean 

 electric current induced by magnetic disturb- 

 ances. 



joint — A fracture in a rock, generally vertical or 

 transverse .to bedding, along which no appre- 

 ciable movement has occurred. ( 2 ) 



Joule — A unit of energy equal to 10" ergs or to 

 0.2389 calories. (5) 



juvenile water — Water that enters for the first 

 time into the hydrologic cycle. It is released 

 from igneous rocks through volcanic activity 

 at a rate probably not exceeding 0.1 cubic kilo- 

 meter per year. (32) 



juvenile wave platform — A shoal nearshore bot- 

 tom consisting of rock, jagged reefs, cobbles, and 

 boulders and having little material easily mov- 

 able by waves. It represents the stage before 

 planation by wave erosion. 



207-109 O— 66- 



91 



