SPONGES. 57 



often proceeds to grow around stones, or to take in sand until its flesh is full of such 

 indigestible ballast. Such specimens will sometimes be a foot long, and weio-h several 

 pounds. Occasionally this form is found attached in the usual manner, and when the 

 locality is free from stones or sand the specimen is clean and free from such encum- 

 brances. In the Mediterranean this genus plays no smallpart in the disintegration of 

 the limestone rocks of the shores. It is evident that this sponge is a. borer from inclina- 

 tion and not from necessity, and also that the inclusion of sand.and stones is not needed, 

 but is probably due to the effort to become attached to any object within reach. It is 

 difficult to explain the boring except as a chemical process, but no one as yet has been 

 able to detect any acidity in the secretions. It may be, however, that it is accom- 

 plished by the silicious spicules on the surface. That they are not the sole means of 

 boring is shown by the recent observations of Nassonow, who ascertained that the 

 young began to bore before the formation of any skeletal structures. 



The Monactinellidan forms in the palaeozoic rocks are uncertain, though Zittel 

 records Cliona from the Silurian, and two genera in the Carboniferous. The first 

 undoubted forms occur in the Jurassic. 



StJB-OkDBK IV. — POTAMOSPONGI^. 



The Fresh-water Sponges, in our opinion, form a group of sub-ordinal rank. The 

 skeleton is similar to that found in the last group, but a very important difference is 

 found in the reproduction. In these fresh-water forms there are found what are known 

 as winter buds or statoblasts. These are protected by an outer coat of spicules of a 

 peculiar form, wholly unlike anything else found in the sponges. They may be as 

 simple as the spicules of the sponge skeleton, and arranged flatwise in the corneous 

 wall of the statoblast, or they may be shaped like a collar stud (birotulate), and 

 arranged vertically. There seems to be a definite line between these two types, and, 

 in fact the sub-order has been divided into two families upon these characters, 

 and named the Lacustridse and the Fluviatilidae respectively. About ten genera have 

 been described by authors from the fresh waters of all parts of the globe. They are 

 usually green in color when exposed to the light, but when found under stones or in 

 shaded localities are of a brownish hue. 



They have a decided affection for clean water and hard bottoms, being in large 

 part attached to stones, logs or plants, but will grow sometimes on muddy bottoms. 

 In such cases the young anchor themselves to small sticks or stones, and thus secure 

 themselves from being choked by the mud. 



The sponge dies during some cold spell in the autumn, and their quick decay 

 in large quantities is one of the principal causes by which the water supply of even 

 a large city may be vitiated. They seem to be the cause of the peculiar smell 

 known as the "cucumber odoi-," and render the water extremely disagreeable as a 

 beverage. 



The preservation of the species is accomplished by the statoblasts which retain 

 their vitality through the winter, usually enclosed in the skeleton at the base of the 

 colony. They develop in the spring, producing new colonies. Mr. Potts, of 

 Philadelphia, accounts for the large size and rapid growth of the sponges in the 

 spring by the coalescence of numbers of the young which develop within the 

 meshes of the same old base. This author asserts that he has repeatedly observed 

 that the young sponges from the statoblasts build upon the undecayed remnants of 



