MOLLUSCA. 91 
species, among which are ants and bees. They have the mouth furnished 
with mandibles for cutting and tearing, but other parts of it formed for 
suction. The wings are four in number, membranous, not folded when at 
rest, the first pair larger than the second, the wings of the same side 
united in flight by little hooks. Many species of this order have stings, 
and they are the only insects which have. 
Among the orders of insects which undergo complete metamorphoses, 
but, in their perfect state, have the mouth formed only for suction, the 
first that demands notice is the order Lepidoptera! to which butterflies, 
moths, and hawk-moths belong. The mouth has a long trunk, coiled up 
when not in use ; the wings are four in number, membranous, and covered 
With minute closely set scales, of very various forms. Great beauty 
appears in the whole order.—The order Diptera? contains all the two- 
winged insects commonly known as flies, also midges, gnats, &. The 
number of species is immense. ‘The mouth is formed for suction alone. 
The insects which undergo no metamorphosis are comparatively few. 
They are all destitute of wings. Many of them are parasitical on other 
animals. 
Mollusca. 
Coming now to the primary division of the animal kingdom called 
Mollusca, we return to the consideration of animals far lower in organisa- 
tion than those which have last been under notice, and ascend as by a 
new path to higher tribes ; for there is not in nature a regular unbroken 
gradation, as some have fancied, from the lowest to the highest forms of 
animal life. But in the highest Mollusca we have a nearer approach than 
in any of the other animal sub-kingdoms, to the highest of all, that of 
Vertebrata. Many of the lower Mollusca—those forming the order Polyzoa* 
—were until recently ranked among Zoophytes, and they have a strong 
general resemblance to the polypes already noticed as belonging to the 
great group of Celenterata. They are, like them, of minute size, and many 
have a similar coralline fabric. They also resemble them in multiplying 
by gemmation and forming compound animals. In the lowest groups of 
Mollusca, we find only one principal nervous ganglion. In the higher 
groups, there are several ganglia lying somewhat irregularly in different 
parts of the body, and communicating by nervous threads with a larger 
mass. It is only in the highest Mollusca that a distinct head is found, 
bearing organs of special sense. The lowest groups have no distinct 
heart, but all the higher possess it. The bilateral symmetry of form, 
almost universal in articulated and in vertebrate animals, is comparatively 
_1 From Greek Jepis, a scale, and pteron, a wing. 
2 From Greek dis, twice, and pteron, a wing. 
3 From Greek polys, many, and zdon, an animal. 
