194 PROCEEDINGS OF THE 



to be derived from the primitive tetract form by an increase in 

 the number of rays. The second type of microsclere is a very 

 small aster, with short, blunt rays, known as a pycnaster. The 

 third type is the curious form called the Geodia ball, or sterraster. 

 It is a curious spicule, consisting of an oval body, of larger size 

 than either of the others, and practically solid. It has a de- 

 pression on one side, supposed to mark the position of the 

 nucleus of the mother cell. Prof. Sollas has figured the Geodia 

 ball with a nucleus. It consists, like other siliceous spicules, of 

 hydrated silica or opal. The surface is covered with small 

 projections. If you take a section of a Geodia ball — an optical 

 section can easily be focused — you find it is composed of 

 enormous numbers of slender rods, radiating from the centre, 

 and each one terminating in one of the surface projections. The 

 development shows it to be a true aster. It starts as an ordinary 

 aster ; as it enlarges it becomes a form with slender hair-like 

 rays, radiating from the centre ; as they increase they thicken 

 till they meet one another, and finally a further deposit of silica 

 gives rise to the marks on the surface. The arrangement of the 

 spicules is as follows : Taking the megascleres first, we find 

 that if you look at ^ cut surface of the sponge, you notice to- 

 wards the outside a definite radial arrangement ; towards the 

 inside this is confused. The radial arrangement is due to bundles 

 of megascleres, which run towards the surface. The heads of 

 the orthotriaenes are extended beneath the cortex, whilst the 

 shafts run inwards for a long distance. Associated with these 

 are found the anatriaenes. The larger oxeote spicules tend to 

 form very loose bundles, but towards the interior they lose 

 this arrangement. The whole sponge is covered with a white 

 incrustation about l/3rd mm. thick ; this is composed entirely 

 of the Geodia balls. Here we have a case where certain of 

 the microscleres have a definite function, forming a protective 

 armour on the outside. The minute pycnasters are on the 

 extreme outer surface. The oxyasters are scattered throughout 

 the ground substance, between the main spicules of the skeleton. 

 It can be seen that there are canals ramifying through the body 

 of the sponge, but the minute structure of the canal system 

 cannot be determined from the dried specimen. This sponge 

 belongs to the Astrotetraxonida, as shown by the astrose charac- 

 ter of its microscleres. 



