120 THE VOYAGE OF H.M.S. CHALLENGER. 
edge, show plainly a layer next the cavities of the zooids consisting of well differentiated 
fibres disposed transversely to the lengths of the cavities. 
The sclerites or spicules are imbedded in the thick layers of the sarcosome intervening 
between the autozooid and siphonozooid cavities and between the tubes composing the 
stem, and also occur in the tentacles of the autozooids. The growing tips of the spicules 
project amongst the ectodermal cells (Pl. IL. fig. 3), carrying with them their invest- 
ment of connective tissue. When the spicules are removed by acid, corresponding cavities 
are left in the mesoderm. A transparent membrane can be distinctly seen investing 
closely each spicule ; no fibrillar or cellular structure, however, could be seen in the 
membrane. 
The autozooid and siphonozooid cavities, and the whole of the canal system, are in- 
vested as usual by an endodermal layer, consisting of spherical cells with yellow contents 
exactly like those of Heliopora. In the siphonozooid cavities, at their summits, around 
the top of the stomach, masses of these cells were always observed to be accumulated. 
Possibly the accumulation of these in this situation is consequent on action taking place 
on the death and contraction of the colony when placed in spirits. 
Vascular System.—Sarcophyton is an extremely favourable subject for the examina- 
tion of the vascular system. In sections from alcoholic specimens preserved in glycerine 
jelly the whole ramifications of the vessels are most clearly displayed. Owing to 
the pigmentation of their lining endoderm, the canals show out dark and defined in 
the perfectly transparent connective tissue. The arrangement of the canals is shown in 
Plate I. fig. 2. 
Two systems of canals are to be distinguished—the transverse and vertical systems. 
The transverse canals run parallel to the surface of the colony and to one another in each 
interspace between the autozooid cavities. They take the most direct courses to connect 
the cavities of the autozooids with those of the surrounding siphonozooids and with those 
of the adjacent autozooids. They commence to be given off laterally from the autozooid 
cavities at their very summits, forming there communications with the siphonozooid 
cavities. They continue to be given off at tolerably regular intervals, crossing now to a 
closely situate siphonozooid, now to a distant one. Deeper down in the colony the canals 
make long stretches to join the next adjacent autozooid cavity, and become shorter and 
shorter as the autozooid cavities converge below. Similar short canals connect the 
siphonozooid cavities with one another. Running in a general vertical direction between 
these transverse canals are the vertical ones, distinguished by their more undulatory 
course. The chief stems of this system of canals are the direct prolongations of the 
siphonozooid cavities. In connection with these canals is an irregular meshwork by 
which the whole deep connective tissue is permeated, and through the meshes of which 
the transverse canals pass. Offsets of the vertical canal system pass between the siphono- 
zooid and autozooid cavities and give off transverse connecting branches. 
