678 THE INVERTEBRATA 



tains, not an epicardial tube, but a longitudinal septum of mesoderm ; 

 and the internal organs of the bud are formed by the complication of 

 a vesicle which arises by the hollowing out of a mass of mesoderm 

 that comes into being in a swelling at the end of the stolon, (c) In 

 Botryllus and its allies budding is effected in yet another way. These 



c.cl. 



ect. 



bud. sin. 



est. 



Fig. 473. Fig. 474. 



Fig. 473. A diagram of a zooid, with a portion of one of its neighbours, 

 imbedded in an ascidian colony. The shaded area is the common test. bud. 

 newly formed bud; c.cl. common cloaca; cL, cl.' cloacas of two zooids; epc. 

 epicardium; est. endostyle; h. heart; ov. ovary; ph. pharynx; rm. rectum; 

 St. stomach ; stn. stolon ; t. testis ; vas de. vas deferens. 



Fig. 474. Diagrams of the budding of tunicates. A, transverse section of 

 the stolon of a zooid such as that shown in Fig. 473 ; B, similar section of 

 stolon of Pyrosoma or Salpa ; C, part of a transverse section of a zooid of 

 Botryllus. at. atrium ; atr. tube from which atrium is formed ; bl. blood space ; 

 bud. bud arising; ect. ectoderm; end. endoderm; epc. epicardium; est. en- 

 dostyle ; g. strand from which gonads are formed ; mdm. mesoderm strand ; 

 n. strand from which ganglia are formed. 



genera, which, unlike Ciona but like most solitary ascidians, possess 

 no epicardium (epicardial diverticula), form their buds by paired 

 outgrowths that are of quite a diflPerent kind from the stolon, for they 

 arise from the atrial wall and each contains an inner vesicle which is 

 a prolongation of the epithelium that lines the atrium of the parent : 

 this vesicle forms the internal organs as well as the atrium of the bud. 



