OP THE SPONGIADiE. 43 



The variety in the amount of curvature at the middle of 

 the shaft of the spiculum is also very great, as represented 

 in Plate V,- Figs. 109 to 121 ; but these variations are not 

 purely accidental ; on the contrary, they are more or less 

 constant in each species of sponge, and frequently afford 

 good specific characters. 



In the simple bihamate form, where the two hami are 

 curved in the same plane and towards each other, the 

 spiculum, in its natural condition, is usually attached to 

 the surface of the membrane by the middle of the back of 

 the curved shaft, and the two hooks are projected into the 

 sarcode at right angles to the plane of the membrane on 

 which it is based. When the hami are developed reversed 

 or at right angles to each other, one of them is then usually 

 imbedded sideways on the membrane, and the other with 

 the shaft is projected from the plane beneath into the 

 sarcode at various degrees of angle. Or in the deflected 

 form the shaft may be firmly cemented to the membrane 

 by one side, while the hami are both projected upward into 

 the mass of sarcode. In some species of sponge one or the 

 other of these forms especially prevails, but in others, as 

 in Halichondria incrustans, Johnston, the simple, reversed, 

 and contort forms are indiscriminately mixed in the tissues, 

 and they occur in every imaginable form of attachment 

 in great profusion, and accompanied by the anchorate 

 forms as well. 



However varied they may be in form, when they are in 

 their normal positions their office appears to be purely 

 retentive. They are generally produced singly, and are 

 dispersed without any approach to regularity over all 

 parts of the sarcodous membranes of the sponge, abound- 

 ing in some situations to a very much greater extent than 

 in. others. Their positions on, and mode of attachment 

 to, the membrane are exceedingly varied, but in almost 

 every instance it is such as to render the spiculum obviously 

 subservient to the retention of the sarcode on the mem- 

 branes which it covers. In one instance only I have found 

 the simple bihamate spicula congregated in loose fasci- 

 culi. In this sponge, a new and very interesting species. 



