21 ANATOMY AND PHYSIOLOGY 



defence as exhibited in some of the principal genera of the 

 Spongiadse. 



In the external defences, the mode of the application of 

 the spicula depends in a great degree on the structure of 

 the skeleton of the sponge. The most simple cases are 

 those where the structure of the skeleton consists of spicnla 

 radiating from the centre or the axes of the sponge, and in 

 these cases they usually consist of the terminations of the 

 radial lines of the skeleton, the distal spicula of which arc- 

 frequently projected for a considerable part of their length 

 through the dermal membranes, and in many sponges the 

 surface is thus thickly studded with them ; and in species 

 where the terminal radial lines of the skeleton contain 

 many spicula, they are frequently found at their apices to 

 assume a radiating direction, so as to present the greatest 

 possible number of points to their external enemies. This 

 mode of defence is very general in the numerous British 

 species of the genera Isodictya and ChaJina, Bowerbank. 

 Fig. 287, Plate XVII, represents a small portion of a section 

 at right angles to the surface from Ckalina seriafa, Bower- 

 bank, illustrating very distinctly this simple mode of 

 external defence. 



In the genus DidyocyRndms, Bowerbank, which con- 

 sists principally of slender branching sponges, many of 

 which in their living state are exceedingly fleshy in their 

 appearance, the skeleton is formed of a central cylinder, 

 composed of a network of spicula, from the surface of 

 which radiate in vast quantities long, slender and acutely 

 pointed spicula, which in the living condition project 

 slightly beyond the dermal membrane of the sponge, so 

 that in the event of any small fish attempting to feed upon 

 or suck this tempting bait, instead of a mouthful of soft 

 and grateful gelatinous matter, he would find himself 

 assailed in every direction with an infinite number of 

 minute points, many of which he would carry away with 

 him deeply imbedded in the soft lining of his mouth, as 

 the reward of his temerity and a warning against a repe- 

 tition of a like assault. Fig. 365, Plate XXXII, represents 

 a small portion of a young branch of Dictyocylindrmrugosus, 

 Bowerbank, frequently found on shells and stones dredged 



