240 THE ASCIDIANS. 



an independent perforation, so that all those preliminary 

 ontogenetic processes which precede the establishment of 

 the transverse rows of stigmata in Ciona are dropped out 

 of the development of Clavelina.* 



In Clavelina, again, the change of axis of the body 

 proper occurs in the unhatched larva ; so does the fusion 

 of the two atrial apertures to form the dorsal cloacal 

 siphon. The longitudinal muscles of the body proper 

 commence to appear in the free-swimming larva, while the 

 caudal muscles are enjoying their highest functional 

 activity. The vacuolisation of the notochord does not 

 proceed so far as in Ciona, since the cells are never actu- 

 ally removed from the centre of the notochord, but remain 

 as thin discs stretching across the latter, so that the 

 vacuolar spaces do not become continuous. 



The behaviour of the organ of fixation in the larva of 

 Clavelina is such that it could hardly be recognised as a 



praeoral lobe except in the light of Ciona. 







NOTES. 



i. (p. 183.) The test or cellulose mantle of the Ascidians con- 

 tains great numbers of cells of various kinds. These were formerly 

 supposed to be derived from the subjacent ectoderm of the body- 

 wall. KOWALEVSKY has recently shown, however, that the cells of 

 the outer (cellulose) mantle of the Ascidians are derived from 

 wandering mesenchyme-cells which wander from the body-cavity 

 through the ectoderm (either between the ectodermic cells or 

 actually passing through the individual cells) into the mantle. 



* A mode of formation of the branchial stigmata, intermediate between 

 that of Clavelina and Ciona or Molgula, has been described by GARSTANG 

 for Botryllus. In this genus, the primary branchial stigmata all arise by in- 

 dependent perforations, and then later become divided up into the transverse 

 rows of stigmata. (W. GARSTANG. On the development of the stigmata 

 in Ascidians. Proc. Roy. Soc., Vol. LI. 1892.) 



