ATLANT. DEEP-SEA EXPED. 1910. VOL. 111]. 



SCYPHOMEDUSAE 



17 



Pelagia per/a (Slabber). 



Jmm -»^ 



10 



20 



30 



<i0 



SO, 



The "Michael Sars" brought home from 

 the cruise of 1910 more than two hundred 

 and fifty specimens of Pelagia, but it is im- 

 possible to distinguish more than one species, 

 and I have therefore followed the old custom 

 of maintaining the name of Pelagia perla, 

 commonly used for the Atlantic Pelagia, all 

 the more as the specimens agree with previous 

 descriptions of this species. 



Vanhoffen 1 ) has paid great attention to 

 the nettle-warts in his specific diagnoses, and 

 this led me to study their variations in the 

 "Michael Sars" material very thoroughly, in 

 order to see whether a separation into groups 

 might be based on their characters. Every 

 attempt in this direction was in vain ; the nettle- 

 warts are generally roundish or ovate, but some- 

 what larger specimens show all the different 

 variations which have served Vanhoffen and 

 other investigators as specific characters. Fur- 

 ther, the different wart-forms vary greatly in 

 number from individual to individual, but so 

 gradually as to afford no guidance in regard 

 to a division into groups. 



The larger warts are found in the middle 

 of the exumbrella; towards the margin of the 

 bell they are smaller, and in the lappets, or at 

 the very margin, they become almost invisible or disappear 

 entirely. From this general arrangement very few excep- 

 tions could be found, though in some small specimens 

 the warts seemed to be of almost equal size all over the 

 disc. The larger warts of the central parts of the disc 

 are separated by larger spaces than usual, and now and 

 then these spaces attain such dimensions that the central 

 parts of the disc are almost smooth. Another variation is 

 often observed which completely changes the appearance 

 of the warts, for in the larger specimens a coalescence of 

 the larger warts often takes place, the warts forming 

 shorter or longer bands instead of ovate or roundish spots. 



Sometimes the warts all over the medusa fade away 

 almost completely, and can only be traced with the greatest 

 difficulty, while in other cases they are distinctly circum- 

 scribed and very prominent. In this respect every tran- 

 sition stage is to be found, and we are thus compelled 

 to abandon the nettle-warts as affording specific characters 

 in Pelagia. 



The next character to be studied in detail, is the 

 relative length of the manubrium and the mouth-arms (see 

 the accompanying table). Here and there the table indi- 



100mm. diar, 



lengih of tht manubrium. 



of the mou.th-a.rms. 



Fig. 10. Diagram showing lines of correlation in the "Michael Sars" specimens 



of Pelagia perla, between the total diameter of the medusa (horizontal scale) 



and the length of the mouth arms resp. the manubrium (vertical scale). 



cates rather abrupt differences. A careful examination of 

 the individuals shows different degrees of contraction 

 especially of the mouth-arms. This is seen most clearly 

 in large individuals, whose arms though intact vary in 

 length sometimes as much as 25 per cent or more, the 

 short arms being thick and compact, the long arms, on 

 the contrary, thin and relaxed. Smaller specimens also 

 show varying degrees of contraction, although, owing to 

 their smaller dimensions, this is less conspicuous at first 

 sight. The table shows how the manubrium and the 

 mouth-arms increase in length with the growth of the 

 medusa, though at a different rate. Smaller specimens, 

 which are still in Ephyra-stage, or such as have not yet 

 their eight tentacles equally developed, have no mouth- 

 arms at all, or they are merely seen as prolongations of 

 the gastric wall indicated at the four corners of the sto- 

 mach. In most cases the medusa with a total diameter 

 of about 7 mm has mouth-arms of the same length as 

 the manubrium. Later the growth of the mouth-arms is 

 more rapid than that of the manubrium, as shown by the 

 accompanying diagram (fig. 10) giving the lines of corre- 

 lation of growth, drawn from the table of measurements. 



') Untersuchungen iiber semaestome und rhizostome Medusen. (Bibliotheca Zoologica, Bd. 1). Cassel 1888, p. 6. 



