REPORT ON CORALS — HELIOPORID^E. 121 



There is no surface network of canals present in the superficial layer of mesoderm 

 directly beneath the ectoderm, such as exists in Heliopora. 



The large canals in the stem of Sarcophyton, which are in reality long drawn out 

 prolongations of the body cavities of the autozooids, may be considered as corresponding 

 to Kolliker's " sinus ; " the vertical and transverse canal systems to the " canales nutrici 

 majores ; " the network in connection more directly with the vertical systems to the 

 "vasa nutrica minora." Apparently there are no vessels corresponding to the " vasa 

 eapillaria," their place being occupied by the network formed by the small ramified 

 corpuscles in the sarcosome. 



The transverse and vertical canal systems anastomose with one another frecmently, 

 but only here and there. Occasionally, but rarely, the canals from the bottoms of 

 the siphonozooid cavities join directly the transverse canals. The canals have a 

 wall of fibrous tissue directly continuous with the fibrous layers of the siphonozooid 

 and autozooid cavities, and are lined internally by endodermal cells (PI. II. fig. 3, C). 

 Sac-like enlargements or swellings are constantly to be seen on the canals of Sarco- 

 phyton, both near the surface and in the deep tissue. In one such swelling was 

 found a parasitical cyst of oval form and with greenish contents ; its nature could not 

 be determined. 



Disposition of the Dorsal and Ventral Aspects of the Zooids. — The autozooids 

 in Sarcophyton are so disposed that they have the dorsal intermesenterial spaces 

 directed towards the centre of the pileus, and at the verge of the pileus these spaces 

 uppermost. At least this disposition was observed to hold good in three opposed 

 radial directions from the centre of the pileus. A whole specimen was not available for 

 examination. 



As in Heliopora the autozooids are not disposed with perfect regularity in this 

 manner, so that radial lines from the centre of the pileus would pass directly through 

 their longer diameters. Many of them are rotated more or less on their axes, so as to be 

 inclined to the radial lines. They are most regular in disposition at the margin of the 

 pileus. The siphonozooids, though preserving a general uniformity of arrangement, 

 which proves their single pair of mesenterial filaments to be the dorsal ones, are still 

 more irregular in disposition. 



General Remarks. 



Affinities of Heliop>ora. — Heliopora is most undoubtedly an Alcyonarian. The 

 number of its mesenteries, and the distribution with regard to them of the retractor 

 muscles, the form and number of its tentacles, are decisive evidence in the matter ; and 

 this evidence is borne out by almost every item of histological structure. In the peculiar 

 manner in which the retraction of the tentacles takes place, viz., by introversion, Heliopora 



(ZOOL. CHALL. EXP. — PART VIL — 1880.) G 16 



