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



In this specimen the pileus is about 5 cm. in diameter, being somewhat oval in outline ; 

 the height from the bottom of the stem to the summit of the pileus is also about 5 cm. ; 

 the diameter is about 1*5 cm. The colony is of a uniform brown colour. 



On examination the Sarcophyton was found to present many points of interest, 

 especially in comparison with Heliopora ; a short description of its anatomy will there- 

 fore be given. 



Terms. — As in Pennatulids, two kinds of individuals, sexual and asexual, compose the 

 Sarcophyton colony. Such individuals have betn termed by Kolliker polyps and zooids, 

 the latter term denoting the sexless or partly aborted polyps, which act mainly as pores 

 for the regulation of the water supply of the colonies. I propose to term in the case of 

 Alcyonarians, in which there are two kinds of zooids, the sexual forms " autozooids," and 

 the aborted polyps (zooids of Kolliker) " siphonozoids." 



Structure of the Zooids. — The stem of Sarcophyton consists of a series of tubular 

 canals running parallel to one another vertically, and bound together by abundant 

 transparent connective tissue, in which are closely packed, numerous, stout, calcareous 

 spicules of the common elongate cylindrical form, pointed at both ends, and covered 

 with small lateral tubercules. The canals are prolongations of the autozooid cavities 

 from above. The surface of the stem and under-surface of the pileus are covered with 

 an even coat of epidermis, and entirely free from zooids of both kinds. On the upper 

 aspect of the pileus the surface is covered all over with autozooids and siphonozooids. 

 Over the general upper surface the autozooids are pretty evenly distributed at intervals, 

 the interspaces being filled by numerous siphonozooids ; but at the margin of the pileus, 

 where its edge is turned down and slightly recurved, is a narrow zone all round, 

 occupied by thickly set autozooids with very few siphonozooids. In a vertical section 

 through the central axis of the whole colony, the autozooid tubes are seen to be arranged 

 with great regularity, converging in curved or vertical lines, according to position, 

 towards the stem. The circular areas occupied by the retracted autozooids measure 1*4 

 mm. in diameter, those occupied by the siphonozooids "42 mm. in diameter — the differ- 

 ence in dimensions being here much less than in deeper regions of the colony, where the 

 autozooid cavities widen and the siphonozooid cavities contract. There are narrow 

 intervals between the circular areas, in which there project the thickly set tips of 

 spicules, which show through the epidermis (PI. II. fig. 3) and form stiff supports to the 

 walls of the cavities. 



Autozooids. — The structure and relations of the autozooids and siphonozooids are seen in 

 Plate I. fig. 2. The autozooids present no remarkable features ; they have numerous fine 

 spicules in their tentacles, which are, as is usual, simply retracted, and are provided with 

 protractor and retractor muscles. Of the protractor muscles (PM in PL I. fig. 2) part of 

 the fibres appear to be inserted into the wall of the autozooid cavity, whilst others are 

 continued on the inner borders of the mesenteries. 



