I028 PRINCIPLES OF STRATIGRAPHY 



haps the rudiments of the sixth pair. The compound eyes are 

 stalked. Other larval stages follow until the adult is reached. 

 These larvae often occur in vast quantities in the plankton. 



Arthrostraca. This super-order includes mostly marine forms, 

 though the order Isopoda comprises marine, fresh water, and ter- 

 restrial types. The latter occur in damp woods, under moss, stones, 

 or logs, and are also abundant in the crevices of rocky cliffs. The 

 marine Isopods are common in shallow water, on alg?e. or swim- 

 ming about in tide pools. Chiridotca cccca lives on sandy shores, 

 moving about just below the surface of the sand, and leaving a 

 mcshwork of curious trails in its wake. Some of the lower sand 

 Hats are often found covered with these markings. The animal 

 is usually found at the end of the trail, its whereabeuts being in- 

 dicated by a little lump of sand. Lininoria lignonmi eats its way 

 into driftwood or bridge piles, often completely riddling and de- 

 stroying the wood. Caprella clings to hydroids and slender sea- 

 weeds. Though chiefly found in the littoral district, some abyssal 

 species are known, several having been obtained at a depth of over 

 3,600 meters. Sometimes different species of the same genus range 

 from shallow water to great depths. Blind species are common 

 in the Mammoth and other caverns. A number of types are 

 parasitic on fishes. The Amphipods are chiefly marine, though 

 fresh and brackish water species also occur in marine genera 

 (Gammarus). Cyamus is parasitic on the skin of whales. Several 

 species live on the beach just above the reach of the ordinary tide, 

 where they hide under the dead seaweed or luirrow in the sand 

 (Orchestia). They generally move about by leaps and hence are 

 commonly known as beach fleas. Talorchestia lives in a similar 

 manner just a little below high-water mark and the beach at low 

 tide is often completely riddled by its burrows. A number of 

 Orchestida? can live wholly out of the water in damp woods or in 

 the dry stream beds. 



Acerata. 



Merostomata. The Xiphosurans are represented by the single 

 living genus Limulus, which is a marine vagrant benthonic animal, 

 though often swimming on its back when young. The usual habitat 

 of this animal is in shallow water, where it is often partly buried in 

 the mud or sand. Portions of the coast are often strewn with the 

 cast-off exoskeletons of Limulus, which usually lie on their backs, 

 a position which these structures will naturally take on sinking to 

 the bottom. 



