104 ANATOMY AND PHYSIOLOGY 



thus formed appear to differ very slightly from those of 

 Halichondria panicea, in each of which several pores are 

 opened, while those of Bactylocalyx Prattii seem to be 

 devoted each to a smaller number (Fig. 306, Plate XXIX). 



As the ternate spicula thus united for the support of the 

 dermal membrane would afford it little or no protection 

 against the voracity of its smaller enemies, we find the ne- 

 cessary defence in innumerable short, stout, entirely spined, 

 cyhndrical spicula, not exceeding -g^ inch in length ; thus 

 minute, there is no conceiving a predaceous creature with a 

 mouth so small that they would not enter and become a 

 subject of annoyance so great as to interfere seriously with 

 its attacks on the membrane ; and they are so numerous, 

 and so closely packed together, that no portion of it equal 

 in size to the length of a spiculum could be removed 

 without one or two of them accompanying it. 



A still further advance in this system of dermal support 

 and defence is exhibited in the beautiful harrow tissue of 

 Dr. A. Parre's siUceo-fibrous sponge, Farrea occa, Bower- 

 bank, MS., to which his specimen of Euplectella cucumer, 

 Owen, is attached. In this case we have a perfect and 

 regular quadrilateral network of smooth siliceous fibre, from 

 the angles of which a double set of short, conical, spicular 

 shafts are projected, each about ^ inch in length and 

 entirely spined. Each set are at right angles to the plane 

 of the network, one series pointing inward and serving 

 the purposes of attachment to the mass of the sponge 

 beneath, while the other set are directed outward, serving 

 as defensive weapons ; so that a small piece of this tissue 

 beneath the microscope closely resembles an agricultural 

 harrow, with the difference that it has two sets of teeth in 

 opposite directions instead of one. The dermal membrane 

 has been nearly all destroyed ; but entangled with the 

 fibres of the skeleton there are some attenuato-stellate 

 spicula, with which it is probable the dermal membrane was 

 amply furnished as secondary defences against its minute 

 enemies. 



I believe the surface presented to the eye in the portion 

 represented by Pig. 311, Plate XXI, to be the external 



