APPENDIX 229 



tive features, though they have a swim-bladder like the 

 Teleosts. Such are sturgeons, bony pike, the polypterus of the 

 Nile, and so forth. Finally, the Dipnoi or double-breathing 

 fish include only three living forms (Ceratodus, Protopterus, 

 and Lepidosiren), interesting because they possess both lungs 

 and gills. 



The phylum vertebrata includes in addition some other 

 small groups, such as the Cyclostombs or round-mouths (hag 

 and lamprey), the Tunicates or Sea-squirts, degenerate when 

 adult, but almost tadpole-like when young, and so forth. 



Though we commonly use the term invertebrate as a con- 

 venient contrast with vertebrate, there are many phyla of 

 invertebrates not nearly related to each other. The highest 

 is the PHYLUM MOLLUSCA, including forms breathing by 

 gills, usually furnished with a limy shell, having a loose fold 

 of skin called the mantle, and a muscular protrusion, on which, 

 e.g., the snail creeps, called the foot. There are three important 

 classes : 



1. The Cephalopoda or cuttles, marine forms in which the 

 shell is usually lost, active and carnivorous in habit. 



2. Lamellibeanchiata or bivalves, forms like oyster and 

 mussel, with a double shell, usually sedentary in habit. 



3. Gasteropoda or univalves, including snaUs, slugs, peri- 

 winkle, &c., in which the shell is present or absent, and when 

 present has but one valve. 



The very large PHYLUM AETHROPODA includes animals 

 in which some at least of the segments of the body are fur- 

 nished with jointed appendages. There are three main 

 classes : 



1. The Crustacea, mostly aquatic forms, with a hard coat, 

 two pairs of antennae, and giUs, including large forms like crab 

 and lobster, and the small copepods or water-fleas, &c. 



2. Antennata Tbacheata, including insects, millipedes, 

 and centipedes, and the curious worm-like Peripatus, all 

 breathing by air-tubes, and having one pair of feelers or 

 antennae. Most live on land. 



