RESEARCHES INTO THE STRUCTURE OF THE ASCIDIANS 19S 



Lamellibranchiate Mollusk ; in opposition to the former view, he 

 endeavoured to show that the tentacles of the Polyzoa are represented 

 by the tentacles of the Ascidians ; and against the latter, he urged, 

 that the gills of the Bivalve Mollusk have no representative in the 

 Ascidian. The " branchial sac " of the latter, represents not the gills 

 of the Mollusk, but the perforated pharynx of Amphioxus ; an analogy 

 which has already been noticed by many observers. 



The author brought forward the structure of the peculiar genus 

 Appendicularia, as fatal to the view that the branchial sac of the Ascidian 

 is homologous with the united tentacles of the Polyzoa. 



Especial attention was directed to the formation of what the 

 author termed the " Atrium,'' under which term he included the 

 cloaca and the space between the branchial sac and the " third tunic " 

 of writers. The author endeavoured to show that it answers to the 

 mantle-cavity of ordinary Mollusks ; that its excessive development 

 accounts for the presence of the " third tunic " in the Ascidian, and 

 that Savigny's comparison of an Ascidian to an inverted Patella had 

 very considerable justice. 



The author next proceeded to detail many structural points of 

 interest which he had made out in the genera examined. A minute 

 account was given of the structure of the branchial sac in Boltenia, 

 Cynthia, Plialhisia, Sjmtethys, and other genera. The branchial 

 meshes are always true apertures, generally more or less rectangular 

 or oval in shape ; but in one species described they were arcuated 

 or semilunar, so as to give the appearance of spiral vessels in the 

 branchial tissue. 



The structure of the dorsal folds and of the " Endostyle," a 

 structure first noticed as distinct by the author in his memoir upon 

 the Salpce, was minutely described ; and the singular and character- 

 istic variations in form of the peculiar organ of sense — the " tubercule 

 anterieure " of Savigny — were pointed out. 



The " Tubular System," described in the same memoir as a 

 peculiar and unique organ in Salpa and Pyrosoma, was shown to 

 be the form of hepatic organ proper to, and universal among the 

 Ascidians. 



The reproductive system exhibits remarkable and hitherto little 

 noticed peculiarities, which have led the author to distinguish the simple 

 Ascidians into Monothalamous and Dithalamous groups, the section 

 Styela (Sav.) being the type of the latter. Owing to the discovery of 

 a Marsupial Cynthia, that is, of one whose ova pass through all stages 

 of their development in the Atrium of the parent, the author was 

 enabled to lay some interesting embryological facts before the 



O 2 



