CHAPTEE XV. 

 FISHES AND SEA-SQUIRTS. 



Vertebrates and Invertebrates Structure of a sea-squirt Some com- 

 mon forms Characters of fish The saithe, or coal -fish Sea- 

 scorpions, or bull-heads Fishing- frog The lump-sucker and its 

 eggs Shanny, butter-fish, and blenny, their habits and structure 

 The sticklebacks Sand-launces Flounders, plaice, and other 

 flat-fish. 



ALL the animals we have hitherto studied have been 

 J\. without a backbone, or equivalent supporting-rod down 

 the back, have had a ventral instead of a dorsal nervous 

 system, and therefore, because of these and some other 

 reasons, all belong to the INVERTEBRATES. The VERTEBRATES, 

 or backboned animals, are most obviously represented on 

 the shore rocks by the fishes, of which not a few species 

 occur in the deeper pools. But there is in addition a group 

 of animals which, despite appearances, have some claim to 

 kinship with the great Vertebrate stock. These are the 

 sea-squirts, or Tunicates, which in larval life have a more 

 than superficial resemblance to tiny tadpoles. In adult life, 

 on the other hand, they diverge very widely indeed from 

 the Vertebrate ideal, being little more than sacs of jelly 

 with a tough, transparent coat. 



Tunicates are common everywhere between tide-marks, 

 but the majority are small, so that we may have to hunt for 

 some time before finding a specimen of suitable size for a 

 first essay in dissection. In some shady nook, or under an 

 overhanging rock, you may find a flat, shapeless mass, 

 attached by one of the flat sides, and of a general greenish 

 tint. Peel it cautiously from the stone, disregarding the 

 sudden jet of water by which it shows resentment of the 



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