[18 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 em. 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 been termed by Kélliker 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 
Aleyonarians, in which there are two kinds of zooids, the sexual forms “‘ autozooids,” and 
the aborted polyps (zooids of Kélliker) ‘ 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. I. 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. L. 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. 
