REPORT ON THE DEEP-SEA MEDUSAE. 117 



forms the bottom of the deep coronal furrow. The coronal area of the subumbrella 

 in Atolla consequently corresponds exactly to the umbrella corona of the exumbrella, 

 and is likewise divided into three zones : the inner zone of the internal coronal 

 muscle, the middle zone of the external coronal muscle, and the outer zone of the 

 marginal lobes. The broad, strongly-developed coronal muscle (" musculus coronaris ") 

 consists in Atolla, as in Collapsis (System, taf. xxviii.), of two separate sharply- 

 defined halves. The inner coronal muscle (" musculus coronaris internus," mc) 

 is 5 mm. broad, thin and delicate, and extends like a veil over the inner zone 

 of the coronal area of the subumbrella. It leaves the proximal third of this zone free, 

 as it does not extend as far as the coronal furrow ; it forms at the same time the 

 proximal third of the coronal pouches, which occupy the greater part of this zone. The 

 outer coronal muscle (" musculus coronaris externus," mc") is only 4 mm. broad but 

 extremely thick ; like the outer muscle it consists entirely of circular muscular fibres ; 

 these are accumulated in many layers one above the other in such a way that they repre- 

 sent a band-shaped circular muscle, 2 mm. thick. This extremely strong fleshy mass 

 belongs to the most powerful muscular formations hitherto observed in the Medusae 

 (comp. the transverse section, fig. 4, left, mc", and figs. 7, 8, mc"). The 19 to 22 

 deep radial furrows of the subumbral under surface, which correspond to the tentacles, 

 divide the outer coronal muscle into the same number of sections (figs. 2, 3). Whilst 

 the sharply-defined external coronal muscle forms the middle zone of the coronal area 

 of the subumbrella, its external zone occupies the corona of marginal lobes ; at the 

 subumbral side of each lobe we find a weaker longitudinal muscle, which radiates 

 into the thin membranous and folded marginal border, the patagium. 



The umbrella cavity in Atolla is very small, corresponding with the flatness of the 

 disk. As the wide oesophagus reaches to its opening and fills its axial space, the 

 umbrella cavity actually merely consists of the narrow, circular, hollow space, between the 

 external wall of the oesophagus and the corona of genitalia. Between the four perradial 

 mesenteric folds of the stomach (ivr) it is depressed in the form of four conical niches 

 projecting inwards, which may be considered interradial funnel cavities, although only 

 of small extent and depth (figs. 1,3, ii). 



The gastrovascular system (figs. 3-6) of Atolla is closely allied in many and important 

 respects to that of the foregoing Nawphant a, but still shows several peculiarities which 

 remind us partly of the Tesseronise (Peromedusae), partly of the Semostomas. Of the two 

 principal sections, the axial principal intestine is very simply formed, in the shape of a 

 quadrangular, depending oesophagus, whilst the peripheric coronal intestine shows very 

 complicated formations, and extends in the form of a horizontal corona of pouches, 

 communicating with the axial intestine by four perradial gastral openings. 



The central principal intestine (" gaster principalis," figs. 3-6, g) forms a short, wide 

 quadrangular oesophagus, which hangs freely from the central part of the umbrella disk, 



