DEVELOPMENT. 



87 



more than one osculum is often regarded as a case of incomplete budding. 

 More unequivocal cases are, however, furnished by the external budding found 

 in Thcnea, Tcthya, Lophocalijx, Polymastia, OscareUa, etc., and the internal 

 budding, known as the formation of gemimilcs, found in fresh-water and in some- 

 marine sponges (Topsent). Gemmules are masses of parenchyma cells containing 

 yolk grains and surrounded by a shell composed of a thick cuticular layer, to 

 which silicious structures are often added. The shell possesses an aperture or 

 micropyle, and the whole structure is to be regarded as a portion of the mesoderm 

 cut off from the rest of the sponge. In SpongiUa the shell contains the 

 characteristic spicules known as amphidiscs (Fig. 78, 9). 



That the power of asexual increase and repair of lost parts is probably a 

 widespread phenomenon in the group is indicated by the fact that sponges can 

 be propagated by artificial fission. It has been attempted, with a certain amount 

 of success, to utilise this property for the purpose of increasing the number of 



a 



FIG. 76. Sections through three stages of the development of Halisarca (Oscc.reUa) lobularis 

 (after C. Heider). a, gastrula after its fixation ; I, formation of mesoderm ; c, development 

 of the osculum (Os.), and of the flagellated chambers ; E, inhalent pore. 



marketable sponges in the Mediterranean and in other sponge-growing seas. 

 The marketable sponges belong to the species Euspongia officinalis, the Turkey 

 or Levant sponge ; Hippospongia equina, the horse sponge ; and Euspongia 

 zimocca, the zimocca sponge. They occur all along the Mediterranean coast to 

 a depth of 200 fathoms, and in many other parts of the world. The sand found 

 in new sponges is an adulteration to increase the weight. 



Sponges are found all over the world and at all depths of the 

 ocean. One family only (Sponyillidce) is found in fresh-water. The 

 Tetradinellida are found in deep and shallow water; hut when in 

 deep water, generally near land. The Monaxonida, which comprise 

 by far the greatest number of living sponges, also cling to the land ; 

 while the HexacUnellida, though found in the middle of the great 



