THE PHYLUM CHORDATA 



55 



Colonial Ascidians (Ascidiae Compositse). All of the colonial 

 ascidians are produced by budding of a single parent individual. It is 

 usual to find a group of ascidio- 

 zooids grouped together in such a 

 way that they have a common 

 cloacal opening (Figs. 26 and 27). 

 Naturally also the tunics of the 

 various individuals form a common 

 matrix in which they appear to be 

 embedded. The so-called "sea- 

 pork" from our Atlantic coast is a 

 typical example of that fixed type 

 of colonial ascidian, which forms 

 incrusting masses on rocks and 

 piles and gives the appearance of a 

 piece of pinkish fat pork, per- 

 forated at intervals by groups of 

 pores, which are the mouths and 

 atrial pores of groups of zooids. 



Free-Swimming Colonies of 

 Ascidians (Ascidiae Luciae). 

 These are pelagic forms in which the 

 ascidiozooids are arranged in such 

 a way that the mouth opening is on 

 the outside of a hollow cylinder 

 and the atrial opening on the inside (Fig. 28). Since only one end of 

 the cylinder is open, a steady current of water is forced out at this end 



c.cl. 



-t. 



FIG. 26. Surface view of two sys- 

 tems of colonial ascidians of the species 

 Botryllus violaceus. cl, cloaca or com- 

 mon atrial funnel; or, oral funnels of 

 the individual zooids. (From Herd- 

 man, after H. Milne-Edwards.) 



FIG. 27. Sectional view of a portion of a colony of a colonial ascidian, (Dip- 

 losoma) showing: c, cl, common cloaca into which atrial openings of all zooids 

 empty; br, branchial apertures or oral funnels of individual zooids; t, test or 

 tunic forming the common matrix of the colony. (From Herdman.) 



