CALCAREA. 37 



MEGAPOGON VILLOSUS. 

 (Plate XXXVI., Figs. 115-119.) 



There are four specimens of this new species in the collection, but only one is 

 perfect, the other three being broken pieces only. All are white as preserved in 

 alcohol, and are covered with a dense mat of very long oxea. The following 

 description applies to the perfect specimen (see Fig. 117). 



The sponge is in the form of a nearly straight circular tube, a good deal distorted 

 at the base, where it has its maximum diameter. It tapers gradually to the short 

 oscular collar which terminates the large gastral cavity. 



The structure of the body-wall is shown in Fig. 115 ; it gets gradually thinner 

 towards the oscule. 



Canal System. The dermis is pierced by numerous ostia which lead into 

 large incurrent canals whicli extend radially inwards nearly through the body-wall. 

 The excurreut system consists of large radial chambers or canals which do 

 not appear to be branched, each opening through a large port into the gastral 

 cavity. 



The flagellated chambers are very variable in size and shape; they are mostly 

 more or less thimble-shaped, opening by apopyles directly into the excurrent chambers 

 round whicli they are grouped. Some of the flagellated chambers are nearly spherical, 

 some are so long that they reach almost across the whole thickness of the wall ; these 

 long chambers are often branched. 



Skeleton. The skeleton of the body-wall is made up of the chiactines 

 which reach completely through it and often project, and a few scattered triradiates 

 whose basal rays also often project. It is pierced at variable angles by the very long 

 projecting oxea. All these oxea are broken in the specimens ; the longest fragment 

 measured is 15 mm. long, f'hiactiues occasionally occur at the top of the excurrent 

 chambers, and may thus appear to lie in the middle of the thickness of the wall. There 

 is no gastral skeleton proper; the only supports to the gastral wall (see Fig. 118) arc 

 the paired rays of the chiactiues, amongst which an- ,-i few minute oxea, some smooth 

 and some spined. The dermal skeleton is formed of a thick layer of triradiates 

 (Fig. 116) lying in all directions, with a few minute oxea standing semi-erect 

 round the ostia. 



Oscule. The oscule is at the end of an oscular collar about 1 mm. long whicli 

 has the appearance of a thin, almost transparent extension of the body. The skeleton 

 of the oscular collar consists of quadriradiates lying tangentially on the inside, with 

 the basal ray downwards and the apical ray projecting into the gastral cavity. Ont-ide 

 are triradiates, also regularly arranged, with the basal ray downwards. The edge is 

 made up of both types of spicnle, with a few special oxea which lie horizontally, but 

 do not project beyond the edge, which has no fringe. The junction between the 

 oscular collar and the body-wall is very similar to that shown in Fig. 110. 



