Fig. 64.—A Phosphorescent Marine Infusorian (Noctiluca), magnified
Fresh-water pools and marshes provide such an abundance of infusoria that the animals are generally obtained for study from these sources, and a few of the common and most interesting species inhabiting fresh water have already been described in a former work. Nevertheless, the sea is abundantly supplied with representatives of the class, and it is certain that the beautiful phosphorescence sometimes observed in the sea at night is in part due to the presence of luminous infusoria, some of which appear to have an aversion to sunlight, retiring to a depth during the day, but rising to the surface again after sunset.
CHAPTER VIII
BRITISH SPONGES
It seems to be the popular opinion that sponges are essentially natives of the warmer seas, and it will probably be a surprise to many young amateur naturalists to learn that there are about three hundred species of this sub-kingdom of the animal world to be found on our own shores. It must not be thought, however, that they are all comparable with the well-known toilet sponges in regard to either size or general form and structure, for some of them are very small objects, no larger than about one-twentieth of an inch in diameter, and some form mere incrustations of various dimensions on the surfaces of rocks and weeds, often of such general appearance that they would hardly be regarded as animal structures by those who have not studied the peculiarities of the group.
Sponges are known collectively as the Porifera or Polystomata, and constitute a separate sub-kingdom of animals of such distinct features that they are not readily confused with the creatures of any other group. Their principal characteristic is expressed by both the group names just given, the former of which signifies ‘hole-bearing,’ and the latter ‘many openings’; for in all the members of the sub-kingdom there are a number of holes or pores providing a means of communication between the body cavity or cavities and the surrounding water. Most of these holes are very small, but there is always at least one opening of a larger size at the anterior end.
It will be seen from what we have just stated that sponges exhibit a distinctly higher organisation than the protozoa described in the last chapter, inasmuch as they possess a permanent body-cavity that communicates with the exterior; but in addition to this there are many points of differentiation of structure that denote a superior position in the scale of life.
In order to ascertain the general features of a sponge we cannot do better than select one of the simplest forms from our own shores. If we place the live animal in a glass vessel of sea water, and examine it with a suitable magnifying power, we observe a number of minute pores scattered over its whole surface; and a much larger opening at the free end. The animal is motionless, and exhibits no signs of life except that it may contract slightly when touched. The water surrounding the sponge also appears to be perfectly still, but if we introduce some fine insoluble powder, such as precipitated chalk, or a drop of a soluble dye, the motion of the suspended or soluble material will show that the water is passing into the sponge through all the small pores, and that it is ejected through the larger opening.