SYNASCIDIA. 



35 



act as retractors ; pharynx small, with 3 or 4 (rarely 6) rows of stigmata ; 

 gonads alongside intestinal loop, ova large, single testis round which vas 

 deferens is coiled spirally ; gemmation from pyloric region, thorax and 

 abdomen formed from separate buds ; larval gemmation feeble. 



The budding* (Fig. 28) in the Didemnidae is very remarkable. The 

 buds appear to arise in two separate portions from distinct parts of the 

 body ; the one of these, known as the thoracic bud (6), arises as a diverti- 

 culum of the outer wall of the lower end of the atrial cavity, on the right 

 side opposite the stomach ; the other, called the abdominal bud (c), is a 

 diverticulum of the oesophagus which also projects towards the right side. 

 The thoracic bud is formed of outer ectoderm, ectodermal lining of the 

 atrial cavity and interposed mesoderm (like the bud of the Botryllidae) ; 

 it gives rise to the thoracic portion of a new zooid, i.e. to the pharynx, 

 atrial cavity, rectum and a portion of the oesophagus (Fig. 28, B). It 

 separates from its place of origin and acquires new relations ; these are 

 as follows : the mouth opens on the surface of the colony, the atrial aper- 



lu. 28. Three successive stages in the budding of Didemnum (from Delage and Herouard, 

 after D. Valle). parent ; b pharyngeal bud ; c abdominal bud ; d atrial aperture ; oeso- 

 phagus ; r rectum ; s mouth of bud. 



ture into the common cloaca of the colony, the portion of oesophagus joins 

 the oesophagus and the rectum joins the rectum of the parent. The abdominal 

 bud meanwhile has formed itself into a loop connected at both ends with 

 the oesophagus of the parent ; one end however separates itself from the 

 oesophagus, joins the rectum close to the point of union of the latter with 

 the rectum of the thoracic bud ; and the whole loop forms itself into a new 

 oesophagus, stomach and intestine. A new heart and pericardium are 

 formed in the abdominal bud. Later when the new pharynx and new 

 intestine have developed so as to be equal in size to those of the parent, 

 they become detached from its oesophagus and rectum and join up in such 

 a way that the oesophagus and rectum of the abdominal bud become 

 continuous with the oesophagus and rectum of the thoracic bud. Such 

 is this extraordinary phenomenon. The details are not yet fully worked 

 out, and we await future observations for a more complete understanding 

 of it. It sometimes happens that the two parts of the bud are not equally 



* Delia Valle, Mem. R. Accad. Lincei (3), 10, 1881, and Arch. Ital. Bio- 

 Jogie, 3. 1883. Caulery, Comp. Rend., 1895-97. Salensky, Naples Mit., 

 li, 1895. 



