NATURE ADRIFT 



to the pond skaters. Several species are found in the Pacific and hidian 

 Oceans but only one, H. iiiicaiis{¥ig. 19; 4), is found in the tropical Atlantic. 

 It is most unlikely to be found in the temperate seas round Europe and is 

 merely mentioned here as of passing interest. 



Although the Echinodermata (starfishes and sea urchins etc.) are an 

 entirely marine phylum, only one, Pclagotliitriii, is planktonic though they 

 have planktonic larvae (p. 90). 



The next group to consider is the Tunicata; a sub-phylum of the Chord- 

 ata to which the fish and mammals also belong. They are rather jelly-like 

 creatures that look too simple to be placed so high in order, but which 

 are not so simple as they look. They possess internal gill slits, an endostyle 

 and an epipharygeal groove which morphologically bears a resemblance to 

 the primitive notochord or tirst rudiments of what grows into the backbone 

 of the vertebrates. Some are attached to rocks — the sea squirts — but others 

 are planktonic and fall into two distinct orders, the Appendiculata (or 

 Larvacea) and the Thaliacea. The Appendiculata (Plate XIX: Fig. 23 ; i and 2) 

 are all very small creatures, with a blob of a body and a muscular tail which 

 is often brightly irridescent. By means of glands called 'oikoplasts' they 

 secrete a relatively enormous balloon of jelly called their 'house' and in which 

 they live. This house is delicately punctured in definite patterns and the animal 

 inside lives by filtering off only the minutest cells of the plankton, the nano- 

 plankton, which pass through the pores of the house and become stuck to a 

 rather sticky band which in turn passes them on to the stomach. The 

 commonest species in the British area are the Oikopleiira dioica, which has 

 separate individuals of each sex; Oikoplctira lahrcidoriciisis, a more oceanic 

 species in which both sexes are in the same animal (hermaphrodite) ; and a 

 third smaller and hammer-headed species called Fritilhnia borcalis. 



There are three main families in the Thaliacea each having points of 

 special interest. All are truly oceanic and arc therefore useful indicators 

 of the ocean currents (Chapter 1 1) and are rarely found in places like the Irish 

 Sea or southern North Sea. One of these families is the Salpidae. Salps are 

 cylindrical or spindle shaped blobs or barrels of jelly, usually between 

 ^ inch and 2| inches in length, that could easily be mistaken for jellyfish 

 in a casual glance, but they contain a definite pattern of muscles and a dis- 

 tinct stomach which is usually a prominent dark green ball-shaped structure. 

 There are many species of tropical and sub-tropical salps, but only one 

 cosmopolitan species, Salpa fiisiforiiiis (Fig. 23; 5 and 6), is found in the 

 north-temperate Atlantic areas, and even as fan north as Norway and Ice- 

 land. Iliica (isyiiiiiu'trica is also fairly common in the open ocean but is so 

 delicate that it rarely reaches our shores in recognizable shape. The other 

 species are warm water invaders. One of these lasis zouaria (Plate XXXIX) 



76 



