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 Dipnot 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 CycLosToMEs or round-mouths (hag 
and lamprey), the TuntcaTEs 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 CepHALopPopa or cuttles, marine forms in which the 
shell is usually lost, active and carnivorous in habit. 
2. LAMELLIBRANCHIATA or bivalves, forms like oyster and 
mussel, with a double shell, usually sedentary in habit. 
3. GASTEROPODA or univalves, including snails, slugs, peri- 
winkle, &c., in which the shell is present or absent, and when 
present has but one valve. 
The very large PHY LUM ARTHROPODA 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 gills, including large forms like crab 
and lobster, and the small copepods or water-fleas, &c. 
2. ANTENNATA TRACHEATA, 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. 
