POLYZOA. 411 



close resemblance to the " polypary " of the Sertularian 

 Zoophytes. In the Ascidian Molluscs or Sea-squirts (Tuni- 

 cata), the animal is simply enclosed in a leathery or car- 

 tilaginous case, in which calcareous matter is very rarely 

 developed. Hence we need feel no surprise that the Tuni- 

 caries are not known in the fossil state. The Lamp-shells 

 and their allies (Brachiopoda) possess a bivalve shell consist- 

 ing of two pieces or " valves," which are more or less highly 

 calcareous. Coming to the higher Mollusca, the true Bivalve 

 Shell-fish (Lamellibranchiata), as their common name implies, 

 have also a bivalve shell ; but this is distinguished from the 

 shell of the Brachiopods by sufficiently good characters. No 

 Lamellibranch is destitute of a shell, and the remains of this 

 class occur more or less abundantly in all deposits except 

 the most ancient. The ordinary Univalve Shell-fish (Gas- 

 teropoda), as indicated by their common name, have usually 

 a shell composed of a single piece or " valve." In many 

 Gasteropods, however, there is either no shell at all, when 

 the animal is said to be " naked " (as in the Sea-slugs), or 

 the shell is quite rudimentary, and is concealed within the 

 mantle (as in the ordinary slugs). In other Gasteropods, 

 again (viz., in the Chitons), the shell is " multivalve," con- 

 sisting of eight pieces or valves placed one behind the other. 

 Most, however, of the " multivalve " shells of older writers are 

 really referable to the Cirripedia. In the minute Oceanic 

 Molluscs, or " Winged Snails," which form the class Ptero- 

 poda, the animal is sometimes naked, but is more usually 

 protected by a symmetrical glassy shell, which is always 

 univalve. In the class of the Cephalopoda, finally, great 

 diversity exists in the character of the skeleton. All the 

 ordinary Cuttle-fishes have an internal skeleton, embedded 

 in the mantle, and not visible externally. This internal 

 skeleton may be calcareous or horny, and it may be of 

 a very complicated nature ; but it merely serves to sup- 

 port the soft parts of the animal, and it does not form an 

 external case in which the animal lives. In one Cuttle- 

 fish only (viz., the Argonaut or Paper Nautilus) is there an 

 external shell, but the nature of this is quite peculiar, and 

 it cannot be compared with the shell of any of the ordinary 



