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 (PI. II. 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 consecpient on action taking place 

 on the death and contraction of the colony when placed in spirits. 



Vascular System. — Sarcojohyton 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. Punning 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. 



