514 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



Muscular LAMELLiE. 



The skeletons part of the sponge is divided from the askeletous 

 portion by a membrane, which extends down from the outer surface 

 to the bottom of the groove. This membrane is situated just in 

 front of the points of the connecting fibres (Plate 37, fig. 1) and 

 surrounds the part of the sponge occupied by the large lacunae of the 

 exhalent canals. This membrane consists of radially disposed 

 parullei, elongate, spindle shaped cells. These measure 0'032 in 

 length and 0-0015 mm., in thickness, in the centre. The ends of 

 these cells are very slender forming extremely find threads. In 

 tlie middle of the length but not in the axis of the cell the nucleus 

 is situated, it lies near the side and is very elongate, oval situated 

 longitudinally. It measures 0"0034 mm., in length and 0-0011 mm., 

 in width. The cell is entirely filled by a very granular and highly 

 colorable pi-otoplasra. The granules are remarkable for their large 

 size, which may be estimated at 0-00025 mm. The ground substance 

 in which these granules are imbedded does not refract the light 

 very much, the granules however are highly refracting. The ground 

 substance refracts the light simply, whilst the granules show when 

 examined with the polariser that they refract the light doubly in a 

 similar manner as the discs in the straited muscles of higher animals. 

 These granules are scattered throughout the cell in an irregular 

 manner, but still one notices that they appear to have a tendency 

 to group themselves in ti-ansverse rows or discs. In this way 

 the cells of the muscular membrane of our sponge appear as 

 an interesting transition form from the simple undifferentiated 

 contractile elements of sponges to the striated muscular cells of 

 higher Ccelenterates. It will be known to the reader that striated 

 muscular elements, which are so widely distributed in the latter 

 and have even been found in Hydroid Polyps (1), do not occur in 

 sponges. The cells in this organ of our sponge are the nearest 



(1) R.v.Lendenfeld. Muscular Tissues in Hydroid Polyps. Proceedings 

 of the Linnean Society of New South Wales. Vol. IX., Pt. 3, p. 635. 



