44 ANATOMY AND PHYSIOLOGY 



Uymedesmia Zetlandica, Bowerbank, they occur in great pro- 

 fusion. Very few of them occur singly ; nearly the whole 

 of them are found in rather loose fasciculi, and the number 

 is generally so great in each as to render it very difficult 

 or impossible to count them. The mode of their dis- 

 position in the bundles is symmetrical, all the hami being 

 in the same plane and coincident in direction, as repre- 

 sented in Fig. 296, Plate XVIII, bundles of reversed 

 bihamate spioula was observed, and these in like manner 

 were coincident in every respect like the simple bihamate 

 ones. 



The type of this form of spiculum, the simple bihamate, 

 is not peculiar to the Spongiadse; it occurs in a much 

 more highly organised class, in a radiate animal, Echinus 

 sphcsra, Forbes, ' British Starfishes,' where we find an 

 abundance of these organs disposed on the external surface 

 of the tubular suckers of the animal, but they are composed 

 of carbonate of lime instead of silex. I am indebted to 

 my friend, the late Mr. John Howard Stewart, for my 

 knowledge of this interesting fact. 



From the simple bihamate forms there appears a pro- 

 gressive development through the uniclavate and biclavate 

 forms represented by Figs. 118, 119, and 120, Plate V, 

 and the unipocillate and bipocillate forms represented by 

 Figs. 123, 124, 125, 126 and 127, Plate V, to the fully 

 developed anchorate forms of spicula. 



In the simple form of pocillated bihamate spicula, the 

 terminations of the curved shaft resolve themselves into 

 two nearly equal, circular, concavo-convex plates, the 

 convex surfaces being in each case outward, and the sides 

 of each plate curving considerably towards the other, their 

 planes being at a right angle to the axis of the shaft. In 

 other cases, one cup will be developed with its plane in the 

 same direction as the axis of the shaft, while the' other cup 

 is produced with its plane at right angles to the axis, and 

 also of the plane of the first cup. In these variations 

 of development, therefore, this form of spiculum may be 

 compared to tne simple and contort forms of bihamate 

 spicula ; and in truth they differ from them only in this. 



