ZOOPHYTES. 27 



ORDER IV.ASCIDIOIDA. 



THERE is among the inoUuscous or soft -bodied animals, which 

 in popular language are known as " shell-fish," a numerous 

 order in which the animals are covered, not with calcareous 

 shells, but with a soft membranous covering or tunic, and 

 are hence called tunicated mollusca. Among them is a genus 

 bearing the name of "Ascidia," one species of which is 

 everywhere abundant round our coast. To this the Zoophytes 

 of the present order bear, in some points, such a resemblance 

 in structure, that the name "Ascidioida" is employed to 

 denote the likeness. 



Fig. 15. PLUMATELLA. 



These Polypes are not separated, but aggregated; their 

 polypidoms are very variable, both in form and in material ; 

 sometimes enamelling with delicate net-work the frond of a 

 seaweed or the exterior of a bivalve shell, at others rising 

 into the aspect of miniature plants, or broad leaf-like expansions. 

 They are furnished with distinct orifices for the reception of 

 food, and for throwing off its undigested remains (Fig. 15). 

 Round the mouth is a circle of retractile tentacula covered with 



Fig 1 5. a, natural size. b, a group in various positions, magnified. 



