78 



ASCIDIANS 



CHAP. 



(genus Anurella, Lacaze-Duthiers), the embryo does not become 

 converted into a tailed larva, the development being direct 

 without metamorphosis (see Fig. 42, C). The embryo when 

 hatched gradually assumes the adult structure, and never shows 

 the features characteristic of larval Ascidians, such as the 

 urochord and the median sense - organs. Fig. 4 2 shows an 

 Ascidiid (A), a Cynthiid (B), and this exceptional Molgulid (C), 

 type of larva, and three forms of Compound Ascidian larvae, the 

 Distomatid (D), the Botryllid (E), and the Diplosomatid (F). 



Fig. 42. — Larvae of various Ascidians. A, Ascidia mentula, Linn. ; B, Polycarpa 

 glomerata. Alder ; C, Anurella roscovita, Lac.-Duth. ; D, Distaplia TJiagnilarva^ 

 Delia Valle ; E, Polycyclus renieri, Larak. ; F, Diplosomoides lacazii, Giard. 

 (Mostly after Lahille. ) 



In the Molgulidae the viscera are characteristic in position 

 and appearance. The alimentary canal lies on the left side of 

 the branchial sac, and the intestine forms a long narrow loop 

 directed in the main transversely. The pericardium and heart 

 are on the middle of the right side, and behind them is placed 

 the single sac-like ductless renal organ, generally occupied by 

 one or more concretions. The gonads are in most cases on both 

 sides of the body, in front of the intestine on the left, and in 

 front of the heart on the right ; but in Eugyra there is no gonad 

 on the right side, and in some other forms the gonad on the left 

 side is absent. (For Oligotrema, see p. Ill, note.) 



There are a number of British ]\Iolgulidae, the two commonest 



