38 HEADLESS MOLLUSKS. 



they present are almost infinitely varied. Some, as the 

 salpse and ascidioe, are simple isolated beings, having no 

 organic union with others of their race ; but one group 

 consists of aggregated beings — a multitude of individuals 

 being organically united together, and thus forming 

 a common vital mass ; and in this respect they ap- 

 proach the zoophytes, or coral-forming polypes, except- 

 ing that they have no external or internal calcareous sup- 

 port, as their own production. Some of these strange 

 compound beings are luminous ; and when shoals are 

 seen floating at night on the surface of the ocean in the 

 warmer latitudes, they present a most brilliant spectacle. 

 In order to give a general idea of the structure of the 

 tunicata, let us take one of the ascidicB as an example, 

 of which many species are met with on our shores. 

 We find it to be a shapeless being, fixed by a base to 

 the surface of a rock or a fragment of coral, and desti- 

 tute of beauty or attraction. It appears to be a helpless, 

 inert, fleshy mass, scarcely claiming a place among living 

 creatures ; but this idea vanishes when its internal struc- 

 ture is investigated, and the laws of its economy are 

 found, so beautiful, so admirable, so full of appeal in 

 favour of the Divine hand by which it was created. 

 We do not, in fact, on looking at this mollusk, see 

 more than the flexible external tunic or bag in which 



