SEA-SQUIRTS. 147 



Nervous System. In the simplest forms, the nervous 

 system consists of a simple cord ganglion, between the two 

 body-openings. 



Development. They multiply by eggs and by budding, 

 the young in some cases assuming at first a free-swimming 

 tadpole form (Fig. 185, a), finally becoming attached, # a , 

 the tail / absorbing, and the animal assuming the adult 

 form B. 



Pyrosoma. The pyrosoma (Fig. 186), or fire-body, is 

 a gigantic compound ascidian, sometimes five feet long, 

 twelve inches across, with walls three inches thick. It is 

 a colony of ascidians forming a cylindrical, barrel-shaped 



FIG. 186. Pyrosoma gigas, a luminous compound ascidian. 



object open at one end. Here the circulation of water 

 aids in locomotion. The mouths, or the inhalent openings, 

 are upon the outside of the pyrosoma, and the exhalent 

 upon the inside. Each individual of the colony is con- 

 tinually engaged in drawing in water, sifting out the parti- 

 cles of food, and ejecting it from the interior opening, all 

 these little currents finding their way out of the single 

 large opening, the combined volume forcing the entire 

 colony along. 



NOTE. The pyrosoma is one of the most wonderful animals of the 

 sea in its illuminating properties. In the southern seas they resemble 

 bodies heated to a white heat, illuminating the greater depths, so that 

 the forms of sharks, porpoises, and other larger animals are seen dis- 

 tinctly far below ; and when at the surface light up the sails, masts, and 



