xvin METAMORPHOSIS OF LARVAE 5 I 3 



Polyzoa deserve more special notice. There is generally great 

 difficulty in persuading larvae to fix themselves when kept in a 

 small quantity of water, which becomes over-heated in the air of 

 a laboratory. The difficulty may be surmounted by placing 

 colonies containing embryos, together with some clean pieces of 

 the seaweed on which the adults are habitually found, in a vessel 

 closed by a piece of fine muslin, and by leaving the vessel 

 attached to a buoy or in a deep tide-pool. The larvae being 

 without an alimentary canal, fix themselves, after a very short 

 free life, on the seaweed. 



It is probable that a great struggle for existence normally 

 takes place at the commencement of the metamorphosis. Any 

 one who will examine, in June or July, rocks covered by Fucus on 

 which Flustrella hispida is growing, will probably find numerous 

 young fronds of Fiicus, from half an inch to an inch or two 

 in length, growing under the shelter of the older fronds. The 

 bivalve larvae of Flustrella show a marked preference for fixing 

 on these young fronds perhaps in order that the duration of 

 life of the colony may coincide with that of the Fucus and 

 these young fronds are commonly covered by very numerous 

 recently-fixed larvae, and by young colonies of various ages. Or, 

 it is easy to observe, by placing pregnant colonies of Bowerbankia 

 in a vessel of water, that the larvae, which are hatched out in 

 thousands, fix themselves in dense masses on certain parts of the 

 wall of the vessel. It is clear that but a small proportion of 

 these larvae will find room for further development. 



Next with regard to the mode of fixation. Attachment 

 always takes place by the surface on which the mouth or its 

 rudiment is situated, and the permanent alimentary canal opens 

 on the opposite surface. In Pedicellina, the one case in which 

 the larval digestive organs are known to become those of the first 

 adult individual, this presupposes a rotation of the alimentary 

 canal, in order to bring it into its new position. 



It is well known that the larvae of other fixed animals may 

 undergo a somewhat similar change. Thus those of Ascidians 

 and of Barnacles fix themselves by their anterior end, and ulti- 

 mately reach their adult form by performing a kind of a somer- 

 sault. The process may perhaps be explained by supposing that 

 some part of the anterior end or of the oral surface is specially 

 sensitive, and that the larva fixes itself by that portion of its 



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