6oo 



THE INVERTEBRATA 



A zooid which arises by budding is known as a hlastozooid {hlasto- 

 zoite) : one which arises from an ovum is an oozooid. In the ThaUacea 

 the oozooid differs considerably from the blastozooid, and has lost 

 the power of sexual reproduction. In the Salpida and Doliolida the 

 blastozooid has lost the power of budding, so that there is a regular 

 alternation of generations. 



cxl. 



hi'id. stn. 



Fig. 448. 



Fig. 449. 



Fig. 448. A diagram of a zooid, with a portion of one of its neighbours, 

 in an ascidian colony. The shaded area is the common test. hud. newly 

 formed bud; c.cl. common cloaca; c/., cl.' cloacas of two zooids; epc. epi- 

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

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



Fig. 449. Diagrams of the budding of tunica tes. A, transverse section 

 of the stolon of a zooid such as that shown in Fig. 448 ; 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 ; hi. blood 

 space; hud. bud forming; ect. ectoderm; end. endoderm; epc. epicardium; 

 est. endostyle ; g. strand from which gonads are formed ; mdm. mesoderm 

 strand; n, strand from which ganglia are formed. 



The Tunicata fall into three classes. Of these, one, the Larvacea, 

 only comprises a few little animals which spend the whole of their 

 lives in the larval condition, developing genital organs and repro- 

 ducing without metamorphosis. The other two classes both attain the 



