376 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
on the outside, so that it closely resembles the genus Dendrilla in this 
respect. 
Locality. “Sponges from beneath a floating stage in Suez Docks.” 
Instribution. Red Sea ; Indian Ocean. 
APLYSINA MOLLIS, n. sp. (Pi. 38. fig. 18.) 
This new and rather striking species is founded on six specimens, all 
obtained at one time. The special characteristics of the species are its 
remarkable colour, apparently due to a symbiotic alga, and the very great 
width of the skeleton-mesh. 
The largest specimen in the collection (Pl. 38. fig. 18) is an upright mass 
from which arise three processes at the summit of the sponge. These 
processes are all fairly cylindrical. The other specimens are all much 
smaller and one of them encloses a living Lamellibranch, apparently of the 
genus Avicula. Mr. Crossland’s notes on this species contain a reference to 
this commensalism between the Lamellibranch and the sponge, and he states 
that it is of common occurrence. 
All over the sponge there are slight irregularities of level, protuberances 
and depressions, arranged quite irregularly. 
The surface of the sponge is conulose, and the conuli are very well marked 
and wide apart. They vary from 1 mm. to 2 mm. in height, and are about 
6 mm. or 8mm.apart. They are arranged regularly over the surface, and 
mark the points at which the skeletal fibres reach the surface. They are 
sometimes connected by more or less well-marked ridges. 
The oscula are few in number and irregularly scattered. Two of the 
cylindrical projections from the upper part of the sponge bear oscula on their 
summits. They vary in size from 3 mm. to 10 mm. 
The pores are always situated in small depressions on the surface, and a 
depression may contain one or many pores. In the latter case pore-areas are 
formed, or, at any rate, special aggregations of pores. They are very small 
and numerous, and occur over the whole sponge surface. 
The colour is described by Mr. Crossland as “a very dark green on the 
surface, light yellow where broken.” This colour changes in spirit to a 
uniform deep viclet! This dark green on the outside and light yellow inside 
is also found in Halichondria intricata, Topsent. In Aplysina mollis it seems 
to be due to a symbiotic alga. 
Skeleton arrangemen lb 
The skeleton consists of a few large pithed fibres only, and these fibres 
have very few branches and still fewer anastomoses. At the surface of the 
sponge they are almost always 6 mm. or 8 mm. apart. Their diameter is 
0°3 mm., of which 58; is occupied by the pith. There are no foreign 
bodies. 
