362 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
At the point where the specimen measures 8 mm. in diameter two barnacles 
occur, and at the same point five branches arise from the stem, in varying 
direction and of different size. 
Fragment 3.—A small fragment consisting of a group of nearly cylindrical 
branches, which branch and anastomose with one another. At one point 
there are 3 barnacles close together. ‘The various branches have a diameter 
of from 3 mm. to 5 mm. 
Other fragments, which are for the most part smaller than the ones above 
described, show intermediate conditions between those of these three fragments. 
In some eases strap-shaped branches arise from cylindrical ones, and in one 
case a strap-shaped branch suddenly becomes cylindical in shape. It is 
noteworthy that the barnacles usually grow at the point of junction of two 
or more branches, but sometimes they occupy other positions. 
The barnacles themselves always occupy small cavities, measuring about 
3mm. in diameter, within the sponge. This causes a swelling to appear on 
the surface of the sponge, as noted above, which measures usually about 
6 mm. in diameter, and which has a small aperture at one point by which 
the barnacle is able to protrude its tentacles to the exterior. The inside wall 
of the cavity containing the barnacle is quite smooth and shows no sign of 
either pores and oscula. 
That the chambers in which the cirripede lives are formed by the upgrowth 
of the sponge around it, and not by hollowing out of already formed tissue, 
is clear from the following :—A section cut through the swelling just above 
one of the barnacles, which was lodged in the cleft between two branches, 
showed the following structure. The two branches, and their skeletons, were 
clearly shown in the section, and between them was a mass of tissue notice- 
ably different in skeletal structure, both in size and arrangement of the 
fibres. This seems to render certain that the barnacle settles down on some 
already formed part of the sponge, and that a secondary growth of tissue 
takes place around it. Further, it does not seem possible that the relationship 
of sponge and barnacle should be accidental, for this formation of secondary 
tissue, in scores of cases, to form a well-defined and regular investment for 
the barnacle, always of about the same diameter and always furnished with 
the same small aperture, seems definitely to indicate that the connection of 
the two animals is symbiotic. 
The surface of the sponge is quite smooth, and no projections whatever 
occur on it. 
The oscula are small, few in number, and, as far as can be ascertained from 
the material available, irregularly scattered over the whole of the sponge 
surface. They average 1:0 mm. in diameter. 
The pores are very numerous, and frequently occur in rows above the 
inhalant canals. No definite pore-areas are marked out, however. 
The colour in spirit is a brownish white. 
