SUB-KINGDOM MOLLUSCA. 187 



1. Neuroptera (Dragon-flies, White Ants, May-flies, &c.) Devonian. 



2. Orthoplera (Cockroaches, Crickets, Locusts, &c.) Carboniferous. 



3. Coleoptera (Beetles). Carboniferous. 



4. Hymenoptera (Bees, Wasps, Saw-flies, Ants). Jurassic. 



5. Hemiptera (Aphides, Field-bugs, Cicadas, &c.) Jurassic. 



6. Lepidoptera (Butterflies and Moths). Tertiary. 



7. Diptera (House-flies, Flesh-flies, Gnats, Crane-flies, &c.) Tertiary. 



8. Tkysanura (Spring-tails). Late Tertiary (in amber). 



CHAPTER XVIII. 

 SUB-KINGDOM MOLLUSCA. 



POLYZOA. 



SUB-KINGDOM MOLLUSCA. The Mollusca comprise the ani- 

 mals ordinarily known as Shell-fish, from their commonly pos- 

 sessing an exoskeleton or shell. The Molluscs are soft-bodied 

 and destitute of any evident segmentation. Commonly the integu- 

 ment secretes a hard calcareous or horny envelope, but this may be 

 absent. The alimentary canal is always present, and never com- 

 municates with the body-cavity. The nervous system consists 

 typically of three pairs of ganglia, disposed in a characteristically 

 scattered manner ; but in the lower forms a single ganglion alone 

 is present. A heart may or may not be present, and there may or 

 may not be distinct respiratory organs. 



As a matter of course, it is only with the shell of the Mol- 

 lusca that the palaeontologist has to deal, and those forms 

 which are destitute of this structure are wholly unknown in the 

 fossil condition. The special characters of the shell will be 

 treated of in speaking of the separate classes. In the mean- 

 while it is sufficient to draw attention to some general consi- 

 derations. In the Sea-mosses and Sea-mats (Polyzoa\ the 

 animal is compound, and the hard structures secreted by the 

 colony would not come under the common designation of 

 " shell." In these cases the investment of the colony would 

 rather be termed a " polypidom," and when of a horny nature, 

 it does indeed show a very close resemblance to the " polypary" 

 of the Sertularian Zoophytes. In the Ascidian Molluscs or 

 Sea-squirts (Tunicata), the animal is simply enclosed in a 

 leathery or cartilaginous case, in which calcareous matter is 

 very rarely developed. Hence we need feel no surprise that 

 the Tunicaries, with one or two very problematical exceptions, 



