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 Allman 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 Polyzoa. By Professor Huxley, on the other hand, and by many other authorities, 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 corresponding to the respiratory apertures of these same animals.
* These cases have been, however, otherwise explained, and asserted to be an abnormal mode of sexual reproduction, the solitary and chained individuals not being the offspring of each other, but being the older and younger progeny of the same parent.
Upon the whole, the systematic position of the Tunicata must be looked upon as still unsettled ; though they are generally referred either to the Mollusca or to the "Worms." A few naturalists regard them either as a special group intermediate between "Vermes" and Vertebrata, or as actually belonging to the latter sub-kingdom.