232 THE VOYAGE OF H.M.S. CHALLENGER. 



deep canals. On the left-band, side of the figure portions of the plates of 

 hard tissue forming the lateral walls of the tubular cavities are shown (A, A), 

 with their natural upper margin. Two systems of canals are seen in section 

 near the surface of the coral. The most superficial canals (V, V, V) lie almost 

 immediately beneath the external ectodermal layer; they are more numerous 

 and much smaller than the deeper canals (V, V), which form communications 

 between the adjacent tubular cavities passing over the summits of the lower 

 parts of their walls as is seen on the right-hand side of the figure. Both sets 

 of canals are lined with endodermal cells. 



A A. Portions of the walls of the tubular cavities. 



C T. Calcareous tabulae. 



P. Projecting points of calcareous tissue. 



E. External layer of ectoderm. 



EN. Endoderm. 



C. Mesodermal layer of homogeneous connective tissue. 



D. Layer of connective tissue cells. 



T. Tentacles introverted, seen in longitudinal section. 



T\ Tentacles introverted, viewed directly into their mouths. 



S. Cavity of stomach. 



E M. Retractor muscle. 



M F. Mesenterial filament. 



T C. Tubular cavities of coenenchym. 



V. Superficial smaller vascular canals. 



V. Deep larger vascular canals. 

 Fig. 2. Section vertical to the upper surface of Sarcophyton sp., showing three autozooids 

 and a number of siphonozooids. 



The autozooids are represented in the contracted condition; they occupy 

 three large elongate cavities in the general transparent sarcosome. The 

 tentacles here are not introverted but simply retracted. The sarcosome 

 between the autozooid cavities is traversed by an elaborate network of canals 

 belonging to two systems, a transverse one, and a vertical one, which, how- 

 ever, freely anastomose. The tranverse canals lead directly from one 

 autozooid cavity to another, with a course nearly parallel to the surface plane 

 of the Sarcophyton, or from the autozooid cavities to the siphonozooid 

 cavities. The vertical system of canals has a tortuous, branching, freely 

 anastomosing course. The siphonozooid cavities contract at their lower 

 extremities, and pass directly into this system of canals. Prolongations of 

 the vertical system of canals pass up to the surface between the siphono- 

 zooid cavities, and between these and the autozooid cavities. 



