MOLLUSCOIDA: tUNICATA. 291 



attached to various submarine objects, and consisting of 

 numerous zooids arranged in star-shaped groups. They are 

 almost always "very small, soft, irritable, and contractile, 

 changing their form with the slightest movement." — (Stark.) 



Homologies of the Tunicata. — The general resemblance 

 between a solitary Ascidian and a single polypide of a Polyzoon 

 is extremely obvious ; each consisting of a double-walled sac. 

 containing a freely suspended alimentary canal, with a distinct 

 mouth and anus, and a nervous ganglion placed between the 

 two. The chief feature in the Tunicata, as to the exact nature 

 of which there is much difference of opinion, is the branchial 

 or respiratory sac. By Professor AUman this is believed to be 

 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. 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 {Lamellibranchiata), 

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

 ponding 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. Ascidia Branchialia. 



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

 the body ; intestine lying on one side of it. \Ascidiadce, Botryllus, 



Order II. Ascidia Abdominalia. 



Alimentary canal completely behind the branchial sac, which is 

 comparatively small. (Clavellina, Doliolum, &'c^ 

 Order III. Ascidia Larvalia. 



Permanent larval form. (Afpendicularia. ) 



The following subdivisions are those adopted by Mr Wood- 

 ward : — 



Class Tunicata. 



Fani. I. Ascidiada (Simple Ascidians). 



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

 united ; branchial sac simple ; or disposed in (8 — 18) deep and regulai 

 folds. 

 Fam. II. ClavelUnidce (Social Ascidians). 



Animal compound, fixed ; individuals connected by creeping tubular 

 prolongations of the common tunic through which the blood circulates 



14 



