370 



their structure, are different in most cases to those previously described. 

 They are stout acerate forms, or large, with terminal trifid or triradiate 

 arms, which last play a great part in Geoclla and Pachymatisnia. The last 

 two, also, are similar in forming a hard crust on the outward sur- 

 face, composed of globular spicules of silex, in the one case, ovate, 

 which formed in the interior, seems to be ultimately collected on the der- 

 mis. In Geodia the triradiate headed spicules support this outer crust, over 

 which, however, is a membrane with small stellate spicules, which also 

 abound in the sarcode of the sponge (vide Brit. Spongiadeae, Vol. i., PL 

 xxviii.) Tethya cranium is a very compact mass of acerate spicules, ra- 

 diating towards the outer surface, with trifid terminal spicules projecting 

 through the dermis. Others of this genus indulge in large stellate spicules 

 in the sarcode, but this particular species is noted for a curious and small 

 sigmoid spicule, irregularly shaped. Paehymatisma, unlike the two last, 

 whose outward form is either globular or sub-globular, is somewhat irregular 

 in its massive character. It is, however, very distinctly allied to Geodia in 

 the crust of ovate spicules of its dermal surface, Avhich are likewise sup- 

 ported by a similar radiation of triradiate headed spicules, having long 

 shafts with acerate terminals. Long acuate spicules are dispersed in its 

 skeleton without much order in the reticulation. Its ovate spicule, and the 

 absence of any stellate form, separate it from Geodia and Tethya. Ecio- 

 nemia is allied to Paehymatisma, but has no such external rust. But as it 

 is not a common form one may only remark upon one of its flesh spicules as 

 being doliate, or barrel-shaped. Leaving the farther consideration of this 

 and other less known forms to an advanced student, one may now pass the 

 order, Silicea, for that, which Naturalists consider as the higher group of 

 sponges, viz., the Calcarea. 



None of the Calcarea, at least among British sponges, are found of a large 

 size. Mostly they are very small, and are distinguished by the prevalence 

 of the triradiate spicule in every genus, which form, though not absent, is 

 rare amongst siliceous sponges. The first genus is Grantia, taking its 

 name from Dr. Grant, an eminent worker in this department. Of this 

 Grantia conqwessa has given the largest size. It derives its specific term 

 from its compressed condition, as seen in the matured state ; but in young 

 examples you see no such character, and know it only by other details. The 

 genus has a central channel or cloaca, by which its excreting system is dis- 

 tinguished. The walls of this, in G. eompressa, besides triradiate spicules 

 with long terminal shafts, has a remarkable ciavate acuate spicule, its clavate 

 end, bending at an obtuse angle, on the outward surface of the sponge ; 

 this will at once distinguish the species. G. ciliata is another interesting 

 species ; tube-like in shape, having its sides fringed with a series of ciliated 

 apertures, and the same at the outfall of its cloaca. It is built up of tri- 

 radiate spicules, but the ciliated forms are made up of long slender acerate 

 spicules, tied together at their bases with another acerate form, short and 

 stout. The young condition of this is not so generally simple in shape as it 

 becomes when in a more developed condition. 



Leucosolenia (i.e. white pipe), is a very interesting genus. It is formed of 



