CH. Ill] LIFE IN THE SEA 67 



difficult to find) he will find a guinea inside 1 Some of the dog- 

 fishes {Acanthias, the spur-dog, is the best known) are oviparous 

 and the young are born alive, and on issuing into the sea at once 

 form part of the nekton. 



But most of the other fishes of the North-Atlantic ocean 

 produce eggs which are pelagic, that is they are lighter than sea 

 water, and when they are shed by the parent they rise to the 

 surface, or near to the surface, and there they drift about until the 

 embryo hatches out from the shell. These have absolutely no 

 powers of locomotion and they are drifted about passively by tides 

 and currents, the very type of planktonic organisms. After a 

 period of incubation which lasts for a variable time — one week to 

 three — the larva emerges. It is at this stage one of the feeblest 

 of marine creatures, and though when seen in the watch-glass of 

 the planktologist it is a large and active creature, so far as 

 microscopic animals go, it is carried about wherever winds and 

 currents drift it. The superficial water drift may transport it to 

 regions remote from that in which it was spawned, and countless 

 millions must be borne into the brackish water at the mouths of 

 rivers, where they probably perish, or they may be stranded on the 

 shore. But after a month or so the little fish has grown con- 

 siderably, and now ensues the period of metamorphosis, during 

 which the larva takes on the adult characters and, in the case of 

 most edible fishes, sinks to the bottom of the sea to pass the rest 

 of its life, or, if its parent is a pelagic fish such as the herring or 

 mackerel, it begins its career as a member of the pelagic nekton. 



Fishes are the only representatives of the true vertebrates 

 which appear in the plankton. But there are still the ubiquitous 

 Ascidians, animals which hang on to the vertebrated skirts as 

 degenerate and poor relations. Such are the sea-squirts, and the 

 brilliantly coloured compound ascidians which we find on the 

 weeds and stones of the beach or dredge up from deeper water. 

 The more primitive tadpole-like Appendicularians — " tails which 

 wag heads " — are nearly always present in the plankton, each 

 inhabiting its gelatinous house (Hans). These are permanent 

 members of the drifting category of organisms, but the eggs of 

 the other ascidians form very frequently occurring objects in the 

 plankton. Fishes and ascidians — chordates in strict zoological 



