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



nearly six times as broad as high ; it is from 60-70 (more exactly 58-6G) mm. in 

 diameter, by 10-12 mm. in height. It is very firm in consistency, like cartilage. The 

 exumbrella (fig. 1, fig. 4, right half) is divided by a broad, very deep coronal furrow (ec) 

 into a central umbrella disk and a peripheric umbrella corona. The corona surrounds 

 the disk as a wall does a fortress, or an atoll, a circular zone of coral reefs, does the 

 island it encloses, from which it is separated by a ring of lagoons. The central disk 

 (" discus umbralis," uc) is flat, smooth, circular, and gelatinous ; its radius is nearly twice 

 as great as the breadth of the umbrella corona, and therefore amounts to two-thirds of 

 the whole radius of the umbrella. Its margin is divided by 19 to 22 radial indentations 

 or incisions ("sulci radiales," es) into the same number of notches or teeth. These 

 notches of the disk are quadrangular, 5 mm. broad and nearly as long ; their height at 

 the external perpendicular fall into the "mural ditch" amounts to 8 mm. The teeth lie 

 in the same radii as the rhopalar pedalia (uo) ; they alternate with the tentacular pedalia 

 (>it), which correspond to the radial furrows between the teeth. 



The exumbral coronal furrow ("fossa circularis," ec) in Atolla is so deep, that in its 

 fundus the central umbrella disk is only connected with the peripheric umbrella corona 

 by a very thin gelatinous ring (fig. 4, ec). Its depth amounts to 6-7 mm. ; its greatest 

 breadth (in the lower third) to 4 mm. It resembles a circle of lagoons, which separates 

 the central island from the surrounding atoll reef, or the deep ditch which separates the 

 enclosed fortification from its circular wall. But as the teeth of the umbrella disk, pro- 

 ject towards the outside with their upper edge over the mural ditch, like overhanging cliffs, 

 the upper, cleft- shaped passage into the coronal furrow appears only 1-2 mm. broad. 



The peripheric corona of the umbrella (" corona umbralis ") is half as broad as the 

 radius of the central umbrella disk, and is composed of three different zones : an inner 

 zone of tentacular pedalia (ut), a middle zone of rhopalar pedalia (uo), and an outer zone 

 of marginal lobes (I). The inner zone consists of from 19 to 22 gelatinous sockels of the 

 tentacles ("pedalia tentacularia," ut). They are thick, almost dice-shaped gelatinous 

 pieces, which lie close together and are only separated by shallow radial furrows. The 

 lateral length of this gelatinous die amounts to nearly 6 mm. Each pedalium forms the 

 basis or socket of a tentacle, which springs from its outer surface. On closer con- 

 sideration we perceive the following conditions of form : — The upper (aboral) surface 

 (fig. 1, ut) is smooth, arched rather convexly and hexagonal ; of the six side lines of this 

 hexagon, the inner are contiguous to the coronal furrow, and lies opposite the radial furrow 

 (es) between each two adjacent teeth of the central disk (er). The two inner lateral 

 lines of the hexagon are contiguous to the corresponding lateral lines of the two adjacent, 

 whdst of the three external lines, the middle touches the tentacle basis, and the two 

 outer lateral lie opposite the two adjacent rhopalar pedalia (uo). The lower (oral) 

 surface of the tentacular pedalia has the form of a parallel trapeze, and forms the upper 

 wall of a tentacular pouch. Their inverted lateral surfaces are separated by a radial 



