770 
DE. J. S. BOWEEBAA^K ON THE ANATOJMT 
brane 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 repetition of a like assault. 
Fig. 11, Plate XXIX. represents a small portion of a young branch of JDictyocylindrus 
nigosus^ Boweebank, MS., an undescribed British species, frequently found on shells and 
stones dredged 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 complicated. 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 Tetliea cranium. The distal 
ends of the skeleton fasciculi, composed of large fusiformi-acerate spicula, 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 termi- 
nations are spread widely out in a circle around the lower part of the skeleton fasciculus, 
so as to form a strong and most etficient conical buttress to sustain it in its proper posi- 
tion, 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-ternate ones accom- 
panying 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 (fig. 12, 
Plate XXIX.). The same system of defences prevails also in Tetliea simillimus^ Bowee- 
BAXK, MS., from the Antarctic 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 Tetliea muricata, Boweebank, MS. , the skeleton fasciculi are not protruded beyond 
the surface, but immediately beneath it we find the heads of numerous large furcated 
expando-ternate spicula, with remarkably long and acute terminal radii, wdiile the 
dermal membrane is profusely furnished with attenuato-elongo-stellate spicula. 
In Tetliea Norvegica and Ingalli, Boweebank, MS., and in T. Lyncurium, Johnston, 
the same protection is attained in a different manner. Instead of the spicula of the 
skeleton fasciculi gradually converging towards a point, they diverge considerably as they 
approach the surface, so as to present an infinite number of minute and nearly equi- 
distant points, and in addition to these the dermal memhrane and the coriaceous coat 
of the sponge is supplied v/ith an infinite number of closely packed stellate spicula. 
In some species of the genus Geoclia the system of external defences is still more 
