There are six species of Chaetognatha that are usually taken in Scottish plankton samples from the appropriate localities. These are

Sagitta setosa J. Müller, from lower salinity water such as that found in the southern and central North Sea and the Irish Sea.

S. elegans elegans Verrill, from mixed oceanic and coastal water.

S. serratodentata Krohn, from surface and subsurface warm oceanic water of fairly high salinity.

S. maxima (Conant), from cold deep water.

Eukrohnia hamata (Möbius), from cold deep water.

Sagitta lyra Krohn, from warm deep water.

All are valuable indicator species (Meek, 1928; Russell, 1935, 1939; Fraser, 1937, 1939). Of these, the first five have occurred in somewhat varying numbers and distribution according to hydrographic conditions every year since at least 1935, except possibly during the period 1940–45 when no investigations were made. S. lyra has been taken in most of these years, but was not found in 1935, 1936 or 1937. It was, perhaps, most abundant in 1948, and one specimen was taken in that year from the Moray Firth, though its presence there must be regarded as distinctly unusual.

Spadella cephaloptera (Busch) is widely distributed throughout the Scottish area as a bottom-living form and is therefore not included here as planktonic.

There are several other species which occur only occasionally in the Scottish plankton either as individual specimens or in small numbers in specialized localities: for convenience these may be termed unusual species. They are as follows.

Sagitta elegans arctica Aurivillius, which has occurred in several years in the Faroe-Shetland Channel associated with the community characterized by Eukrohnia hamata and Calanus hyperboreus.