11(5 THE VOYAGE OF H.M.S. CHALLENGER. 



Their basal part is strongly thickened and forms a conical tentacle bulb, which fills the 

 interspace between each two rhopalar pedalia, and is inserted with a broader base at the 

 distal side of the tentacular pedalium. This basal part is also hollow and contains the 

 csecal end of the thin tentacular canal, whilst the distal part of the tentacles is solid (as 

 in Nauphanta). A strong longitudinal muscle runs both on the upper and the lower 

 surface of the tentacle. The upper or external, abaxial tentacle muscle (fig. 4, mt') is 

 the shorter and weaker, only occupies the proximal third of the tentacle, and passes 

 to the outer margin of the upper surface of the tentacular pedalium. The lower or 

 internal axial tentacle muscle (fig. 4, mt") is longer and stronger, runs along the whole 

 length of the tentacle, and passes with two very strong, fusiform radial muscles (mk, 

 fig. 3, above, to the right) to the umbral surface of the tentacular coronal pouch (as in 

 PeriphyUa, PL XXIL). 



The sense clubs (" rhopalia," or) in Atolla are quite rudimentary and more slightly 

 developed than in any other Discomedusae hitherto examined, in fact this might be easily 

 overlooked, as their obscure rudiments lie hidden at the distal margin of the rhopalar 

 pedalia, between the basis of the two marginal lobes of a pair. It was only with consider- 

 able trouble that I succeeded in determining their existence ; they alternate regularly 

 with the tentacles, so that their number also amounts from nineteen to twenty-two. 

 Their anatomic nature could unfortunately not be found out on account of their small size 

 and the bad preservation of the umbrella margin in all five specimens ; but as the sense 

 clubs are indubitably in the same position and better developed in the closely allied 

 Colkqms (System, pi. xxviii. figs. 3, 4), there can be no doubt as to the significance of 

 the small rudiments in Atolla. We have probably to do here, as in many other 

 deep-sea animals, with a phylogenetic retrograde formation of this organ of sense. 



The subumbrella (figs. 2, 3, 4) is divided in the same way as the exumbrella, by the 

 deeply incised coronal furrow, into two separate principal arese, which are only connected 

 by the thin gelatinous ring (ec) at the bottom of the coronal furrow. The central area 

 of the subumbrella is therefore the same size as the central disk of the umbrella ; it is 

 formed by the gastrogenital membrane, which reaches as far as the distal margin of the 

 coronal sinus, and contains the stomach in its central part and the corona of eight 

 genitalia (s) and their alternating deltoid muscles (md) in the peripheric part. The 

 deltoid muscles are narrow and slightly developed, especially the four perradial (md') 

 whilst the four interradial appear to be considerably broader (md"). All the eight 

 deltoid muscles in Atolla are triangular only in the distal half, and rectangular in the 

 proximal half (between the genitaba) ; the interradial muscles are inserted on the base 

 line of the cathammal arese (kt), the perradial at the distal margin of the gastral 

 openings (go). 



The coronal area of the subumbrella begins at the distal margin of the genitalia, and 

 is separated from central area by the very thin ring of the gelatinous disk, which 



