Intkoducj'oby. 21 



fundamental spicule in each. These are the (1) Hexactinellida, (2) 

 Tetractinellida, (3) Monactinellida ; in which the predominating spicule, 

 whether its form l)e unmodified or derived, lias respectively six rays, 

 four rays and but a single ray. 



In the Hexactinellida. (Oscar Schmidt), the normal form of the 

 spicular element is a six -rayed star whose rays are of equal length and lie in 

 three planes at right angles to one another. These spicules are termed, with 

 reference to the rays, 1iex(ieti)is, or referring to the three mutually })erpea- 

 dicular axes, triaxons / f njm the latter term the name Triaxoiiia was applied 

 to the order by Sciiulze. The variations in the form of the normal hexactins 

 are manifold, spicules Avith five, four, three and two rays being not infrequent, 

 but it is believed that all are derivatives from, or modifications of, the triaxial 

 type.* These spicules are either isolated and independent, lie side by side 

 forming fascicles, or are coalesced at their extremities forming a continuous net- 

 work. Two inqiortant subdivisions of this order have Ijeen generally recog- 

 nized, (1) the Lyssacina and (2) the Dictyonina. The fonuer is characterized 

 by the isolation of the spicules \vhich are held together only by the sarcode of 

 the sponge or, in i-are instances, by a loose and irregular deposit of siliceous 

 matter; in the latter every arm of each hexactin is united with the corre- 

 sponding arm of an adjacent spicule, the skeleton thus formed consisting of 

 a frame-work Avitli cubical meshes. 



The forms here embraced under the designation Dictyospoxoid.e belong 

 wholly to the suborder Lyssaciim. So far as their spicular skeletons have 

 been acertained they afford no evidence whatever of the continuous network 

 which characterizes the Dictyonina. 



Symbiosis. Very interesting instances of commensalism occur in some 

 of the living hexactinellids. A well-known case is that of the gorgonian 

 coral, Palyth.oa fatvM, which encrusts the glass rope of Hyalonema. Another 

 is that of the crab inhabiting the paragastral cavity of Euplectella, in which 

 it is imprisoned by the reticulum. As it is stated that the terminal sieve-plate is 

 not developed in the early stages of Euplectella, the crab evidently spends 

 only a portion of its existence thus incarcerated. In regard to this commen- 

 sal, J. E. Gray has written : f " The crab is generally found in the cavity of 

 the sponges. The Spaniards in Manila regard them as formed by the crabs 

 for their protection and they do not consider a specimen perfect unless a crab 

 is contained therein. I have, within the last few days had a pair offered to 



'See Rauff, op. cit. p. 187 ; THOMSO<f, The Atlantic, voL 1, p. 141; aud others. 

 tAnnaU and Magaziuo of N itiiril History, 3.1 ser. vol. 18, p. 489, 1866. 



