OP THE SPONGIADiE. 25 



up at Shetland, or the Orkney Islands. In the genus 

 Tethea, in which the skeleton consists of fasciculi of large, 

 stout spicula radiating from the base or centre of the 

 sponge, the system of defence is somewhat more compli- 

 cated. It is a combination of the terminations of the 

 skeleton fasciculi with, in some species, the addition at the 

 surface of the sponge of porrecto-ternate and recurvo- 

 ternate spicula ; the latter two forms being probably 

 aggressive as well as defensive, subserving the purpose of 

 entangling prey as well as that of defence. 



This mode of defence is very beautifully illustrated in 

 Tethea cranium. Pig. 363, Plate XXXI. The distal ends 

 of the skeleton fasciculi, composed of large fusiformi- 

 acerate spicxila, are projected through the stout coriaceous 

 surface of the sponge, and in the midst of this thick coat 

 each of the passing fasciculi is surrounded by a cluster of 

 stout short fusiformi-acerate spicula, their distal points 

 closely embracing the fasciculus, while their proximal 

 terminations are spread widely out in a circle around the 

 lower part of the skeleton fasciculus at b, so as to form a 

 strong and most efficient conical buttress to sustain it in 

 its proper position, at the same time allowing a considerable 

 amount of elasticity to meet pressure from without. Each 

 skeleton fasciculus terminates with from two to eight or ten 

 porrecto-ternate spicula, and occasionally we find one or 

 two of the recurvo-temate ones accompanying them ; but 

 their apices are rarely projected much beyond the dermal 

 membrane of the sponge, while the rest of the spicula 

 extend considerably above it. The same system of defences 

 prevails also in Tethea similima, Bowerbank, MS., from the 

 Antartic regions; but in this species the recurvo-ternate 

 spicula appear to be protruded in greater numbers, and in 

 more regular order than in our northern species, T. cranium. 



In Tethea muricata, Bowerbank, MS., the skeleton 

 fasciculi are not protruded beyond the surface, but imme- 

 diately beneath it we find the heads of numerous large 

 furcated expando-ternate spicula, with remarkably long and 

 acute terminal radii, while the dermal membrane is pro- 

 fusely furnished with attenuato-elongo-stellate spicula, 

 Pigs. 304 and 305, Plate XIX. 



