172 J. W. FewJces — Deep-Sea Medusas. 



The nature of the 'argument for the existence of medusan 

 life in bathy metrical zones may be best illustrated by consider- 

 ing a few examples of the Acraspeda. These are not the only 

 instances which might be chosen and possibly are not the best. 

 They are thought to be as suggestive as any among the Acras- 

 peda which have been ascribed to great depths. 



One of the most characteristic families of Acraspeda is called 

 the Collaspidce. The family is supposed to belong to the deep-sea 

 and is represented by two genera, Atolla and Collaspis, which 

 differ from each other rather obscurely in the regular or irreg- 

 ular arrangement of the sexual glands. It is a question whether 

 we have more than specific differences in the features which 

 have been pointed out by Hseckel as separating the two. 



Up to the present the genus Atolla \$ represented by a single 

 species collected by the Challenger {A. Wyvillii Hseck.) and two 

 species from the Gulf Stream (A. JBairdii and A. Verrillii 

 Fewkes). 



The structure of Atolla is thought to be more primitive than 

 the ordinary inshore genera, Oyanea and Aurelia. It is so 

 characteristic that I repeat from my paper on the anatomy of 

 this genus, a condensed notice of some peculiarities,* 



If we compare Atolla with our common surface medusae, as 

 Aurelia, we notice many marked peculiarities. 



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

 sented in Aurelia although found in a well known surface 

 medusa (Peripkylla). 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 ITseckel as rudi- 

 mentary, and it is supposed that' we have in a deep-sea medusa 

 an adaptation for a life in the depths into which the light never 



. *The umbrella when seen from the upper side is found to be divided by a deep 

 ring-shaped groove into a central and 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 bearing 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 thin flaps 

 called the marginal lappets. On the under side 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, which opens into a bag-shaped stomach. In the interior of the 

 body there is a circular cavity filling the central disk, which opens by four ori- 

 fices into a ring-shaped sinus which lie in the gelatinous body of the corona. 

 Prom the outer edge of this ring-shaped sinus simple, unbranched, peripheral 

 tubes extend through the bell-substance, passing into the cavities of the tentacles 

 and rudimentary marginal sense-bodies. 



