28 ANATOMY AND PHYSIOLOGY 



same simple but beautiful mechanical contrivances for 

 opening and closing in accordance with the necessities 

 of the animal. For a more complete description of the 

 anatomy and physiology of this highly interesting species I 

 must refer my reader to the ' Transactions of the Microsco- 

 pical Society of London/ vol. vii, p. 79, pi. v. 



In other species of Grantia the same principles of 

 external defensive action exists, but the precise mode is 

 never exactly the same in any two species. 



Their external defences are the homologues of those of 

 the dermis of some of the Holothuriada and of Synapia. 

 Thus in Cucumaria communis we have the dermis fur- 

 nished with an infinite number of beautiful perforated 

 circular plates, from the centre of each of these is projected 

 outward a spiculated umbo terminating in numerous acute 

 points ; when the animal is irritated the whole of these are 

 projected from the dermis and the surface becomes bristling 

 with an infinite number of minute organs of defence. In 

 like manner Synapta is furnished with numerous anchor- 

 shaped spines which lie parallel to the dermal surface while 

 the animal is in an unexcited state ; but when irritated a 

 muscular contraction of the dermis takes place, the shank 

 of each anchorate spine is drawn inward, forming a 

 minute pit or depression, so that it becomes erect, and the 

 sharply pointed flukes, if we may so term them, are brought 

 into defensive position over the whole surface of the body 

 of the animal. 



Internal Defensive Spicula. 



The internal defensive spicula of sponges are exceedingly 

 various in their forms and modes of application to their 

 especial purposes ; and they seem naturally to resolve 

 themselves into three distinct groups : 1st, those which 

 are destined simply to repel; 2nd, those which wound and 

 lacerate as well as repel ; and 3rd, those which are calcu- 

 lated not only to destroy but also to retain intruders. 



The purposes of the first class of spicula are frequently 

 performed by the ordinary spicula of the skeleton, which 

 are projected more or less into the cavities immediately 



