ENTEROPNEUSTA. 237 



The Ascidians are quite common animals of the sea- 

 shore, many of them being found fixed to stones or 

 shells near low-tide mark. If touched, when they have 

 been left dry by the tide, they squirt the contents of 

 the atrial chamber through the mouth and exhalent 

 pore, whence they get the name of Sea Squirt. One of 

 the commonest, often found on oyster-shells, is bright 

 red. Some of the colonial forms are very pretty, the 

 animals being grouped in stars or patterns ; they may 

 often be found on the fronds of tangle, etc. The 

 colonial forms Pt/rosoma and Salpa are highly phos- 

 phorescent. 



Those Tunicates in which the adult retains a tail 

 and a notochord are sometimes placed in a class by 

 themselves, under the name of Pekennichoedata, or 

 of Laevacea, because they resemble the larval forms 

 of the rest of the group. The remainder are sometimes 

 included under the, general name of Caducichoedata, 

 or Tunicates which lose their notochords, and may 

 be divided into two classes, the Thaliacea and the 

 AsciDiACEA. The former are free forms, and have the 

 exhalent aperture placed at the opposite end of the 

 body from the inhalent aperture, while among the 

 fixed forms belonging to the latter the two apertures 

 are near together (fig. 83). 



ENTEKOPNEUSTA OB HEMICHOEDATA. 



Balanoglossus, already named as a worm- like marine 

 animal, which is a link between vertebrates and in- 

 vertebrates, may be placed here for the sake of con- 



