SKELETAL STRUCTURES. 79 



stoma 1 from the Cenomanian of Essen, and I have also observed it in detached 

 spicules of Leuconia from the Pliocene beds of St. Erth. 



3. Four-rayed spicules. These may be described as three-rayed spicules in 

 which an additional ray radiates from the point of junction of the three rays either 

 at right angles or obliquely. This fourth, or apical ray as it has been termed by 

 Haeckel, is, in the fossil forms in which I have noticed it, frequently shorter than 

 the three facial rays of the spicule (Fig. 7, h). Four-rayed spicules are present 

 in the dermal layer of Tremacystia D'Orbignyi* Hinde, and possibly in the dermal 

 layer of other fossil calcisponges as well, but unless the spicules can be isolated it 

 i3 difficult to determine whether a fourth ray is present or not. Four-rayed 

 spicules are also present in the fibres of Sestrostomella clavata* Hinde; in these 

 the rays are curved. 



The Disposition op the Spicules in the Skeleton. 



The manner in which the elementary spicules, whose forms have just been 

 described, are combined together to form the skeleton of the Sponge is very varied 

 in the different groups. According to the nature of this union so is the capacity 

 of the Sponge to resist the disorganizing influences of fossilization, and it probably 

 explains the rarity in the fossil state of certain groups of Sponges which are 

 extremely abundant in the present seas. "We proceed to consider first the skeleton 

 of siliceous Sponges. 



Monactinellida and Tetracti/nellida. In these two groups, which form the large 

 majority of existing siliceous Sponges, the spicular elements of the skeleton are 

 not organically fused together, but are held in their natural positions by an 

 envelopment of a horny substance known as spongin. In some Sponges this 

 connecting substance is reduced to a small amount, which merely surrounds 

 the terminal ends of the spicules, whilst in others the spicules are completely 

 enveloped by it, and thus held together so as to form a meshwork of fibres, 

 in which they are arranged parallel with each other, or they may be grouped 

 in bundles which branch and anastomose, or radiate from the base to the 

 summit of the Sponge. As this connecting horny substance inevitably decays on 

 the death of the Sponge, the spicules become detached and fall apart, and only 

 under very exceptional conditions of preservation does the skeleton retain its 

 natural form in the fossil state. As a matter of fact, entire Sponges of these 

 groups, or even connected fragments of the skeleton, are of the rarest occurrence, 



1 Op. cit., p. 12. 



2 ' Ann. and Mag. Nat. Hist.,' S. 5, vol. s, p. 192, pi. xi, figs. 1—8. 



3 Id., pi. xii, fig. 16. 



