196 THE ASCIDIANS. 



In correlation with this sessile habit of existence we find 

 that the Ascidians, in contrast to Amphioxus, are hermaph- 

 rodite, — an almost universal condition among sessile organ- 

 isms of every description. They are unsegmented, the 

 muscles not being divided up into myotomes ; and none of 

 their organs (gonads, renal organs, etc.) are metamerically 

 repeated, unless we regard the successive transverse rows 

 of stigmata in the wall of the branchial sac as evidence of 

 metamerism. It is, however, of a totally different nature 

 from the metamerism of the gill-slits of Amphioxus, and 

 we shall see that only in the earlier stages of their devel- 

 opment can the stigmata of the Ascidians be compared 

 with the former. 



Another of the most characteristic accompaniments of 

 a sessile mode of life is the U-shaped alimentary canal. 

 Instead of being a straight tube with a posteriorly directed 

 anus as in Amphioxus, the alimentary canal of the Ascid- 

 ians is doubled up upon itself, the rectum is directed for- 

 wards, and the anus opens into the atrial cavity. The 

 absence of a dorsal nerve-tube and notochord in the adult 

 Ascidian has been indicated above. 



In spite of these great differences, the presence of the 

 endostyle and the perforated wall of the pharynx in the 

 adult, and above all the features in the embryonic and 

 larval development, entitle the Ascidians to be defined as 

 more or less Ampldoxus-likc creatures which have become 

 adapted to a sessile habit of existence. 



DEVELOPMENT OF ASCIDIANS. 



The first accurate and detailed account of the embryonic 

 development of Ascidians was the classical memoir pub- 

 lished in 1867 by Kowalevsky in the Memoires de 

 I'Academie imperiale des Sciences de St. Petersbourg. 



