380 MANUAL OF ZOOLOGY. 



but appear to have a much more limited range, being, as far as 

 is yet known, principally characteristic of the north temperate 

 zone. Britain can claim the great majority of the described 

 species of fresh-water Polyzoa, but 'this is probably due to the 

 more careful scrutiny to which this country has been subjected. 

 Fresh -water Polyzoa have also been found in the southern 

 hemisphere, in Australia and India. 



DISTRIBUTION OF POLYZOA IN TIME. The Polyzoa have 

 left abundant traces of their past existence in the stratified 

 series, commencing in the Lower Silurian rocks and extend- 

 ing up to the present day. The Oldhamia of the Cambrian 

 rocks of Ireland, and the Graptolites, have been supposed to 

 belong to the Polyzoa ; but the former is very possibly a plant, 

 and the latter should be referred to the Hydrozoa. Of un- 

 doubted Polyzoa, the marine orders of the Cheilostomata and 

 Cyclostomata are alone known with certainty to be represented. 

 Several Palaeozoic genera such as Fenestella (the Lace-coral), 

 Ptilodictya, Ptilopora, &c. are exclusively confined to this 

 epoch, and do not extend into the Secondary rocks. Amongst 

 the Mesozoic formations, the Chalk is especially rich in Poly- 

 zoa, over two hundred species having been already described 

 from this horizon alone. In the Tertiary period, the Coralline 

 Crag (Pliocene) is equally conspicuous for the great number 

 of the members of this class. 



CHAPTER XLII. 

 TUNIC ATA. 



CLASS II. TUNICATA (Ascidioida). The members of this class 

 of the Molluscoida are defined as follows: "Alimentary canal 

 suspended in a double-walled sac, but not capable of protrusion 

 and retraction ; mouth opening into the bottom of a respiratory 

 sac, whose walls are more or less completely lined by a network of 

 blood-vessels " (Allman). Animal simple or composite. An im- 

 perfect heart in the form of a simple tube open at both ends. Sexes 

 mostly united ; a metamorphosis in development. 



The Tunicaries are all marine, and are protected by a lea- 

 thery, elastic integument, which takes the place of a shell. In 

 appearance a solitary Ascidian (fig. 200, C) may be compared 

 to a double-necked jar with two prominent apertures situated 

 close to one another at the free extremity of the animal, one 



