636 
VAPaOUS KADIATE ANIMALS UPON THE SEA SHOEE. 
the only aperture of the digestive cavity, and is quite destitute oi any masticating apparatus. The 
skin in the compound polypi, which are able to retract themselves into firm cells or tubes, is 
exceedingly soft and tender ; but in the solitary species it frequently acquires a leathery consist- 
ence, forming a closed sac, within which the more delicate tentacles can be retracted at pleasure. 
In many cases the skin contains urticating organs, consisting of minute transparent vesicles, from 
which long spiral threads and a caustic fluid are emitted, which cause a stinging sensation on 
coming in contact with the human skin. Reproduction takes place in these animals both by 
means of ova and by germination or budding. The sexes are always united in the same indi- 
vidual. This class is divided into three orders — Helianthoida, Aateroida, and Hydroida. 
Before we proceed, however, to notice these separate divisions, we must more fully apprise the 
reader of the peculiar nature of some of the animals belonging to this order. It is to be borne 
in mind, that — as we have stated — some polypes are simple and others are congregated — these 
latter being united into one living whole, while, at the same time, each acts for itself; one con- 
tracting, another expanding at the same instant. In one sense we may consider each polype as 
a distinct being; in another sense, as forming a part of a compound unity through which vitality 
is equally diffused. In these aggregated forms the polypes all labor to one end ; they constitute a 
community, every individual of which contributes to the nutrition of the general body. But as 
there are no nerves, they can neither participate in each other's movements or each other's feelings. 
If one polype is destroyed, the rest are unaftected. We may commence our observations by remark- 
ing, that all ideas of life derived fi-om a consideration of the higher animals of creation must here 
be banished. Deadness to pain, yet feeling for light, contractility, expansibility, and motion with- 
out' muscles ; digestion and nutrition without lacteals, absorbents, or blood-vessels ; reproduction 
by simple division or bud-like sprouts ; the vital unity of myriads, and yet their personal distinct- 
ness — these are the curious, strange, anomalous characteristics which belong to the animals we are 
now about to contemplate. 
