16 ZOOLOGY. 
DESCRIPTIVE ZOOLOGY. 
Cuass PoriIrera. 
Ir sponges belong to the animal kingdom, they stand at the lowest point, 
where they will constitute a class to which Grant’s name Porifera may be 
applied. In the living sponges the water is imbibed through the smaller 
pores, and flows out of the larger ones in a regular stream. They exhibit 
no sensation when pierced, torn, burnt, or acted upon by acids, so that they 
are exceeded in sensitiveness by many plants. 
Dujardin considers that he has proved them to be groups of animals. In 
placing a fragment of living sponge under the microscope, it was found to 
shape itself into rounded masses, the edges of which changed their form 
continually ; and small bits moved by contracting and expanding. 
Johnston, in his History of British Zoophytes, classes sponges with plants, 
on the ground that they are permanently fixed, not irritable, their move- 
ments involuntary, a stomach wanting, and from their resembling the 
cryptogamia in taking their form from the object to which they are 
attached. 
Mr. Hogg states that sponges have no tentacles, vibrille, mouth, 
cesophagus, stomach, gizzard, alimentary canal, intestine, anus, ovaria, ova, 
muscles, nerves, ganglia, irritability, palpitation, or sensation. “Surely, 
then, we cannot any longer esteem these natural substances to be individual 
animals, or even groups of animals, in which not one organ or a single 
function or property peculiar to an animal can be detected.” 
Sponges are usually marine, although there are a few species which are 
found in streams and stagnant water. They have a loose texture, covered 
and penetrated by a jelly-like substance; and they are perforated with 
numerous passages. The gelatinous substance seems alone to be present in 
the young, the fibrous substance appearing at a later period. 
The species of spongia are numerous, about 150 kinds being described by 
Lamarck. The best known is Spongia officinalis (pl. '15, fig. 45). It is 
found attached to rocks and stones in the Mediterranean, particularly about 
the Greek islands, where they are collected by divers. Its reproduction is 
so rapid, that it may be collected in the same place after an interval of two 
years. The younger specimens are the most sought after, on account of 
their greater delicacy. Formerly burnt sponge was used in domestic 
medical practice for goitre, its action depending upon the presence of 
iodine. 
The form of sponges is subject to an endless variety, and even the same 
species varies to a great degree, apparently with the locality ; so that it is 
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