A CORAL-REEF AT LOW TIDE. 



CHAPTER IV 

 Lower Forms of Marine Life 



To give an adequate sketch of the invertebrate life of the ocean within the compass 

 of this chapter is an impossibility ; and all that can be attempted is to direct 

 attention to a few — and necessarily a very few — of the more interesting types. 



As constituting a kind of border-land between vertebrates and 

 qmr S ' invertebrates, the so-called sea-squirts, constituting the class Ascidia, 

 or Tunicata, may first claim attention. Looking as they do like animated leather- 

 bottles, and not very highly animated at that, these curious organisms do not sug- 

 gest much of the vertebrate to the ordinary observer ; and, as a matter of fact, it 

 is only in their free-swimming larval condition, when they distantly recall tadpoles, 

 that sea-squirts show any indications of community with the vertebrates, this being 

 displayed by the possession of vestiges of the rod of cartilage running along the 

 back, to which the name of notochord has been applied. In this passage from an 

 active to a passive, and to a certain extent degenerate mode of life, sea-squirts 

 display a feature common to several groups of invertebrates, such as barnacles and 

 oysters. On the other hand, there are certain organisms, such as the feather-stars, 

 which, in place of taking to a kind of arm-chair existence, do just the reverse of 



this, and are free swimmers only when adult. It should be added that a few of 



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