678 i;. KlKKl'ATIMCK. 



The growth of the skeleton is much influenced by tlie 

 conformation of the base. When growing on shells or worm 

 tubes with ridges, the sponge makes squarish oblong meshes 

 with parellel or concentric arrangement (PL 35, fig. 20). 



This last figure of a specimen macerated in Eau de 

 Javelle shows well also the earliest beginnings of the 

 skeleton before bars or tubercles are formed. Here, in 

 places, little smears of calcareous scales are beautifully dis- 

 tinguished on the reddish surface of tlie worm-tube. These 

 glistening bfl,nds have a crystalline appearance as if a brush 

 dipped in strong solution of sugar had been drawn over the 

 surface and allowed to crystallise. The individual scales 

 have a finely punctate structure, and each one shows a little 

 elevation which cori-esponds, I believe, to the nucleus of the 

 cell which, made the scale (PI. 35, fig. 21). Viewed as 

 opaque objects, the fine points and the little nuclear hump in 

 each scale strongly reflect the light. I was unable to detach 

 the glistening scales from the worm-tube so as to examine 

 them by transmitted light under a high power. 



The calcareous skeleton of Merlia is formed of ciilcite. 

 Dr. Gr. T. Prior found that the specific gravity was 2"65. 

 That of pure calcite is 2'7, the difference being due to a certain 

 amount of organic matter remaining in the Merlia even after 

 being finely ground and macerated. The specific gravity of 

 arragonite is much higher, being 2"9, and, moreover, Meigen's 

 test gave no reaction. 



(3) The Soft Tissues and Cells. 



Merlia is semi-transparent, and its tissues are of a soft 

 texture, but, at the same time, rather tough. The bright i-ed 

 colour resides in the granules of certain cells — the amoebo- 

 cytes. The colouring matter is soluble in alcohol, in which 

 it forms an orange-yellow solution. The cells will be de- 

 scribed under the following headings : (a) Collencytes, and 

 ground substance or maltha; (b) gland-cells and cuticle; 



