Mr. J. W. Fewkes on DeejJ-sea Medusae. 253 



One of tlie most characteristic families of Acraspeda is 

 called the Collaspida3. The family is supposed to belong to 

 the deep-sea and is represented by two genera, Atolla and 

 CoUaspis^ which differ from each other rather obscurely in 

 the regular or irregular arrangement of the sexual glands. 

 It is a question whether we have more than specitic differ- 

 ences in the features which have been pointed out by Hteckel 

 as separating the two. 



Up to the present the genus Atolla is represented by a 

 single species collected by the 'Challenger' {A. WyviUii, 

 HaBck.) and two species from the Gulf-stream {A. Bairdii 

 and A. Verrillii^ Fewkes). 



The structure of Atolla is thought to be more primitive 

 than that of the ordinary inshore genera, Gijanea ?^vi.6^ Aurelia. 

 It is so characteristic that I repeat from my paper on the 

 anatomy of this genus a condensed notice of some peculi- 

 arities *. 



If we compare Atolla with our common surface Medusa?, 

 such as Aureiia, we notice many marked peculiarities. 



In the former we have a coronal furrow, which is not repre- 

 sented in Aureh'a, although found in a well-known surface 

 Medusa [Periphtjlla). We have in Atolla a variable number 

 (generally twenty-two) of sense-bodies or peduncles of the 

 same. In Aurelia we have always eight sense-bodies. The 

 coronal muscle is peculiar to Atolla. 



The sense-bodies of Atolla are spoken of by Hseckel as 

 rudimentary, and it is supposed that we have in a deep-sea 

 Medusa an adaptation for a life in the depths into which the 



* The umbrella, when seen from the iipperside, is found to be divided 

 by a deep ring-shaped gTuove into a central and a peripheral region. The 

 groove is called the coronal fossa, the central region the discus centralis, 

 and the periphery the corona. The corona is formed of a number of 

 wedge-shaped gelatinous blocks, joined together and baaring on their 

 outer rim alternately tentacles and sense-organs. These gelatinous blocks 

 are designated by the term socle, taken from architectural nomenclature, 

 and are of two kinds — those which bear the tentacles, called the tentacular 

 socles, and those which carry the sense-bodies (if such exist), the socles 

 of the sense-bodies. The socles of the sense-bodies bear two thiu flaps, 

 called the margiual lappets. On the underside of the disk we have, 

 below the corona, a large ring-shaped muscle, called the coronal muscle, 

 which is highly characteristic and larger in this genus than in any other 

 known Medusa. Axially to this muscle there is a zone formed of eight 

 kidney-shaped sexual glands and a simple mouth, Avhich opens into a 

 bag-shaped stomach. In the interior of the body there is a circular 

 cavity lilling the central disk, which opens by four oritices into a rin"-- 

 shaped sinus, which lies in the gelatinous body of the corona. From tlio 

 outer edge of this ring-shaped siuus simple, unbrauched, peripheral tubes 

 extend through the bell-substance, passing into the cavities of the ten- 

 tacles and rudimentary marginal sense-bodies. 



Ann. & Mag. N. Hist. Ser. 6. Vol. l 18 



