J 6 ZOOLOGY, 



DESCKirXIYE ZOOLOGY. 



Class Porifera. 



If 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 i'lxnn the object to which they are 

 attached. 



Mr. Hogg states that sponges have no tentacles, vibrillse, mouth, 

 oesophagus, 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 witii 

 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 oßdnalis {pi. 75, ßg. 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 

 220 



