MERTENSIA OVUM. 



27 



edges smooth ; the circumscribed area is quite small ; the tentacles are 

 but moderately long, apparently not capable of as 

 great expansion as Pleurobrachia. Our species of 

 Mertensia is exceedingly delicate ; the specimens 

 taken at Nahant and Eastj)ort, though treated 

 with the greatest care, not living more than a 

 couple of hours when brought into confinement. 

 I siippose this to be the Beroe ovum of Fabricius. 

 The difference between the tentacular chy- 

 miferous tubes and the median pairs is very 

 marked in young specimens. In the youngest 

 Mertensia observed, we find the same pear-shaped 

 form noticed in young Pleurobrachia. (See Fig. 41.) 

 The ambulacra, however, are far more advanced 

 in comparison to the tentacles, and occupy nearly 

 the whole of the spherosome (Fig. 30) ; the 

 pouches of the ambulacra concealing almost en- 

 tirely the digestive cavity. There are also very 

 prominent orange pigment-cells, which are not 

 foimd in the young of Pleurobrachia, along the 



rows of locomotive flappers ; the tentacles remain simple much longer 

 than in Pleurobrachia. The young Mertensia is not as much com- 

 pressed as the young Pleurobrachia (compare Figs. 31 and 46, repre- 

 senting almost correspondmg stages of Mertensia and of Pleurobrachia), 



The compression goes on increasing with age, and in the adult it has 

 become one of the striking characteristics of the genus. With advan- 

 cing age the actinal part of the young Medusa becomes more promi- 

 nent, while the ambulacra have remained nearly unchanged, the long 

 and short ambulacra not retaining quite the same proportions they had 

 before they were almost equally developed ; the funnel has become 



Fig. 29. Adult Mertensia seen from the broad side. 



Fig. 30. Young Mertensia seen from the broad side, Tvith a simple tentacle. 



Fig. 31. The same as Fig. 30, seen from the abactinal pole. 



