MOLLUSCA: TUNICATA. 387 



a solitary Ascidian and a single polypide of a Polyzoon is extremely 

 obvious ; each consisting of a double-walled sac, containing a freely sus- 

 pended 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. Pro- 

 bably 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 (Lamellibranchiatd), whilst the oral and 

 atrial apertures are looked upon as corresponding to the respiratory aper- 

 tures of these same animals. 



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 " Verities" and Vertebrata, or as 

 actually belonging to the latter sub-kingdom. 



DISTRIBUTION OF THE TUNICATA IN SPACE AND TIME. 

 The Tunicaries are exclusively marine in their distribution, 

 and are principally littoral and shallow-water forms, though 

 some are found at considerable depths, and many are pelagic 

 in habit. The singular Salpidce have the branchial and atrial 

 apertures placed at opposite ends of the body, and are found 

 swimming in the open sea, often in immense shoals. The 

 Appendicularia, with their permanent larval tails, are likewise 

 oceanic, as is the cask- shaped Doliolum. Lastly, in Pyrosoma, 

 we have a singular compound oceanic Tunicate, in which the 

 numerous zooids form a tubular colony, which is propelled 

 through the water by the united excurrent respiratory jets 

 of its component members. Like the Salpians, it is brilliantly 

 phosphorescent. 



...On the other hand, the more typical Tunicates are found 

 attached to all kinds of submarine objects, or (as in Pelonaia) 

 embedded in mud. 



During the "Challenger expedition," some singular deep- 

 sea Tunicates were obtained, and have been since described 

 by Mr Moseley. One of these (Hypobythius) was found in 

 the Pacific, at a depth. of nearly 3000 fathoms, attached to 

 foreign objects by a peduncle. Its test is hyaline and trans- 

 parent, and is strengthened by symmetrically disposed cartilagin- 

 ous plates. Odacnemus, dredged at over 1000 fathoms, is also 

 hyaline- and transparent, with a short stalk, but it possesses 



