MOLLUSCOIDA I TUNICATA. 3 I I 



truly homologous with the tentacular crown of the Polyzoa, 

 and the oral tentacles of the Tunicaries are believed to be 

 something superadded, and not represented at all in the Poly- 

 zoa. f By Professor Huxley, on the other hand, the branchial 

 sac is looked upon as an enormously developed pharynx, and 

 the oral tentacles are regarded as a rudimentary representative 

 of the tentacular crown of the Polyzoa. Probably the most 

 correct view of the homologies of the Tunicata is taken by 

 Rolleston, who regards the " branchial sac " as the homologue 

 of the gills of the ordinary Bivalve Molluscs (Lamdlibranchiata), 

 whilst the oral and atrial apertures are looked upon as corre- 

 sponding to the respiratory apertures of these same animals. 



DIVISIONS OF THE TUNICATA. By Professor Huxley the 

 following arrangement of the Tunicaries is adopted : 



CLASS TUNICATA. 



Order I. A scidia Branchialia. 



Branchial sac occupying the whole, or nearly the whole, length of 

 the body ; intestine lying on one side of it. (Ascidiada, Botryllus, 

 dry.) 

 Order II. A scidia Abdominalia. 



Alimentary canal completely behind the branchial sac, which is 



comparatively small. (Clavellina, Doliolum, 

 Order III. A scidia Larvalia. 



Permanent larval form. (Appendicularia.} 



The following subdivisions are those adopted by Mr Wood- 

 ward : 



CLASS TUNICATA. 



Fam. I. Ascidiada (Simple Ascidians). 



Animal simple, fixed, solitary, or gregarious ; oviparous ; sexes 

 united ; branchial sac simple ; or disposed in (8- 1 8) deep and regular 

 folds. 

 Fam. II. ClceuettinicUt (Social Ascidians). 



Animal compound, fixed ; individuals connected by creeping tubular 

 prolongations of the common tunic through which the blood circulates 

 (or by a common gelatinous base). Reproduction effected by ova, or 

 by gemmation from the common tube ; the new individuals remaining 

 attached to the parent, or becoming completely free. 

 Fam. III. Botryllidtz (Compound Ascidians). 



Animals compound, fixed, their tests fused, forming a common mass 

 in which they are imbedded in one or more groups. Individuals not 

 connected by any internal union ; oviparous and gemmiparous. 

 Fam. IV. Pyrosormda. 



Animal compound, free and oceanic. 

 Fam. 'V. Salpidcz. 



Animals free and oceanic ; alternately solitary and aggregated. 



