232 
THE VOYAGE OF H.M.S. CHALLENGER, 
deep canals. On the left-hand 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) le 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. 
AA. Portions of the walls of the tubular cavities. 
CT. Calcareous tabule. 
P. Projecting points of calcareous tissue. 
E. External layer of ectoderm. 
EN. Endoderm. 
'. 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. 
RM. Retractor muscle. 
MF. Mesenterial filament. 
TC. Tubular cavities of ccenenchym. 
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. 
