NATURE ADRIFT 



fused to, at most, the first three segments. The abdomen has a series of 

 swimmerets and the tip has a distinct fan tail the inner vanes of which 

 usually contain statocysts or balancing organs. In these a small stone-like 

 concretion or otocyst is free to move about in a sensitive spherical con- 

 tainer, and the position of this otocyst lets the mysid know which way up 

 it is. 



Mvsids are very common in inshore waters along the sandy coast- 

 lines, and especially in estuaries where they move up and down with the tide 

 thus keeping in water of more or less constant salinity. There are oceanic 

 mvsids also, mostly in deep water and some of these are bright red in colour; 

 one species Giiatliopliaiisia (Fig. 22; i) reaches about 3 inches in length. 



Somewhat similar in shape to the mysids are the Eiipliaiisiacea, which 

 after the copepods are the next most important crustaceans in the plankton. 

 Euphausids (Fig. 22; 4, 5) differ from mysids in the carapace which is 

 fused to the whole thorax but leaves the gills exposed below it. They have 

 no statocysts in the tail fan anci they usually have quite prominent lumin- 

 escent photophores under the abdomen between the well-grown pairs of 

 swimmerets. Whilst as a rule not so abundant as the mysids in closely inshore 

 waters they are often extremely common offshore, in the northern half of 

 the North Sea and the Clyde estuary for example, as well as in the open 

 ocean. One species, Eiipliaiisia siiperha, or krill, is by far the most important 

 food for the plankton-eating whales of the Antarctic, while a similar species, 

 Mci^aiiyctiphanes, the northern krill, forms the chief food of the arctic and 

 boreal whales as well as of basking sharks and indeed forms an important 

 part of the food of herring. There are several warm water species that are 

 drifted into temperate waters, some that live naturally in temperate waters, 

 for example Thysauocssa iuermis and Nyctiphanes couchii, and some such as 

 Thysaiioessci loiigicaiidata prefer colder water, and even the northern krill, 

 MeoanyctipJiaiies, although common in places like the Clyde estuary in 

 Scotland, is fundamentally a cool water species. 



The highest order of the Crustacea is the Decapoda. Decapods are 

 mostly bottom-living species, usually with planktonic larvae which will 

 be considered in the next chapter, but the occasional shrimp or prawn is 

 found in the plankton especially near the coasts. A truly planktonic adult 

 is the ghost shrimp Pasiphaea, which sometimes occurs in isolated and local 

 shoals quite near the shore. There are also a number of deep sea planktonic 

 decapods most of which are bright red in life, such as Aauithcpliyra, or other 

 bright colour such as Eryoiwiciis (Plate XVIII). • 



The other classes of the Arthropoda, represented by the centipedes, 

 insects, spiders and mites, all have a few marine forms but none are plank- 

 tonic except for one insect called Halobates, a warm water species allied 



74 



