BV E. F. HALLMANN". '87 



ing the fibres are disposed parallelly to the direction of the 

 fibres, and their number in a cross-section of a fibre at any 

 point is usually about 4 or 5 — though varying from 2 to {very 

 rarely) upwards of 8 or 9. Immediately beneath the dermal 

 membrane, — occurring about the extremities of the fibres, and 

 also scattered in between, — are bundles of short and slender 

 auxiliary (or dermal) styli. Auxiliary styli, in small number, 

 also occur scattered singly in the interstices of the main skeletal 

 reticulation. 



As seen in thin, or moderately thin, longitudinal sections per- 

 pendicular to the plane of the sponge-frond, — owing to the fact 

 that in such sections, as a rule, many or most of the fibres are 

 intersected by the plane of section, — the skeleton often appears 

 as if irregularly isodictyal in pattern, with triangular and quad- 

 rangular meshes the sides of which are of a single spicule's 

 length; and sometimes, in its denser and more irregular por- 

 tions, it appears somewhat halichondroid. The true conforma- 

 tion of the skeleton is generally more plainly and immediately 

 apparent in sections parallel to, and in, the mid-plane of the 

 sponge-frond (PI. xl., fig. 2) ; and such sections, also, display 

 best the structure of the "veins." The pauciserial spicule-fibres 

 originate as branches from the stout fibres forming the "veins," 

 and, proceeding therefrom in a more or less obliquely ascending 

 direction, pass upwards in the mid-plane of the sponge and 

 outwards to the surface, continually multiplying by dichotomy 

 as they proceed. The veins would thus appear to constitute 

 the primary axes of the growth of the sponge. Each vein is 

 composed of several fibres united both by occasional anastomosis 

 and by (numerous.) connecting spicules. These fibres, — which 

 attain a diameter of over 400/x in the oldest portions of the 

 skeleton, gradually diminishing to less than 150/a at the margin 

 of the sponge, — are densely sponginous, and are filled with 

 closely packed styli (similar to those occurring elsewhere in the 

 main skeleton) the apices of which, almost as frequently as not, 

 are directed towards the base of the sponge. 



Megascleres. — (i) The principal or skeletal styli (Text-fig. 

 3. a) are smooth, slightly curved, and of nearly uniform dia- 

 meter usually throughout about four-fifths of their length, 

 •ing thence to a sharp point. As a rule they are very 

 slightly stouter in the middle than at the base. They range 

 from 350 to 53(bx in length and up to 31.5/x in diameter; indi- 



