S6 LIFE IN THE SEA [PART I 



various phases before the full adult characters are acquired by it. 

 The crab zoea moults and passes into the megalopa phase. Both 

 zoea and megalopa were originally described as adult animals and 

 these names are those which were applied to them by their 

 discoverers. So also in the nauplius of the copepods a series of 

 disguised phases may be discerned in the development of this larva 

 into the adult form. The nauplius of the cirripedes (the barnacles) 

 lives in the sea for some time ; then inoults and grows more 

 appendages, and finally a bivalve shell, and again drifts about in 

 the sea for some time before it settles dowai on the objects of the 

 sea bottom as the familiar organism which every one knows. 

 Stomatopods, Ostracods, Isopods, Amphipods, Schizopods and 

 Phyllopods all have their nauplius larva and a series of other more 

 or less easily recognised larval stages appear before the full 

 characters of the species to which they belong are acquired. In 

 the peculiar parasitic Crustacea which infest the bodies of fishes 

 and other marine animals there is a much more complicated life- 

 history, and a long series of larval phases may be included in the 

 process of development by which the creature enters into its 

 definite place in the metabolism of the sea. 



Very few mollusca have a direct development. As a very 

 general rule the Qgg which is spawned into the sea when the 

 breeding season of the shellfish is reached is a minute organism 

 possessing a larval shell and provided with a locomotory apparatus 

 in the shape of a lobe of the body carrying a number of cilia. 

 Cilia are the stiff contractile hairs by means of which the larva 

 of nearly all marine animals progress. This veliger larva is as 

 characteristic of the mollusca as the nauplius is of the Crustacea. 

 It swims about in the sea for a considerable period and then 

 abandoning the planktonic mode of life for a benthic one it 

 settles down on the sea bottom, attached to stones, &c., or in the mud, 

 or in the sand, for the remainder of its life. The egg of the star- 

 fishes and sea-urchins develops into a larva which is as character- 

 istic of these groups as the other larval forms we have mentioned 

 are of their respective groups. The pluteus larva of the starfishes 

 and sea-urchins is an inhabitant of the plankton and when it has 

 lived long enough in this mode of life to become enabled to 

 complete its development it begins to unfold the structure of the 



