184 THE FLOATING ANIMALS 



Of the group of Echinoderms the starfish, brittle- 

 stars, sea-urchins, trepangs, sea-lilies a single adult 

 genus only has as yet been taken in Plankton, but their 

 larvae are frequently captured ; these are even more 

 unlike their adult form than in the Crustacea. The 

 adult form in Crustacea was attained by a series of 

 moults, but in the Echinoderms it rather grows out 

 of the larvae, which, so to speak, shrivels down on to it. 

 Each of the five subdivisions has its own type of larvae, 

 four of which are here shown (Figs. 140 to 147). 



Of the enormous group of Insecta, only one small 

 family is truly planktonic (Fig. 139). 



Of the Polyzoa (they have no English name) none are 

 found in Plankton, except accidentally attached to 

 drift wood or weed. Near shore their larvae are com- 

 mon ; of these, the most likely to strike the eye has a 

 shell like a flattened funnel (Fig. 137). 



The foregoing groups are all taken from the in- 

 vertebrate animals, but there remain some creatures, 

 very common in Plankton, which only a naturalist 

 would dream of classing anywhere near vertebrates. 

 Yet these animals, both by structure and development, 

 are undoubtedly akin to vertebrates ; they have (among 

 other characteristics), either when larval or throughout 

 life, a supporting rod in the back (the notochord), 

 exactly like that of a sturgeon or lamprey, or the 

 similar rod round which the vertebrae are moulded in 

 the embryos of all vertebrates, including Man. The 

 names of these groups refer to this notochord. 



The " sea-squirts " and salps are included in the 

 group Urochordata i.e., with the notochord in the 

 tail. Some of them retain it throughout life ; for 

 instance, Appendicularia (Fig. 148), which can often be 

 captured in this country in an estuary on a flowing 

 tide ; some species form temporary " houses " of jelly- 



