22 



MNEMIOPSIS LEIDYI. 



species the young belonged, as the difference between the diameters is 

 far less marked than in Bolina. As they advance in size, the lobes 

 become developed, the tentacles disappear, and they can be readily 

 distinguished. The development goes on in the envelope for a week 

 or ten days after the eggs are laid, the young Medusa not breaking 

 through the outer membrane before it is well advanced, and capable 

 of guiding its motions through the w r ater. The difference between the 

 two transverse diameters of the spherosome is not as great as in Bolina, 

 as will readily be seen by comparing the broad and narrow views of 

 this Ctenophore (Figs. 22, 23). What is very peculiar in the genus 

 Mnemiopsis is the peculiar development of the tentacular apparatus. 

 It is not, as in Bolina, reduced to a simple bulb, with a few tentacles 

 clustered at the base ; but is more like what we find in Lesueuria, 

 where the threads of the tentacular bulb are quite long, and have a 

 decided tendency to spread fan-shaped on both sides of the bulb. We 



have a rather small tentacular bulb placed 

 at the end of a long, slender tube, a short 

 distance above the opening of the actino- 

 stome (o, Fig. 22). This tentacular bulb 

 is protected by a kind of two-lapped hood 

 (Fig. 24), the folds of which extend on 

 each side along a groove towards the abac- 

 tinal pole, to the very origin of the auri- 

 cles, at ft, Fig. 22, taking their origin at 

 o, Fig. 22 ; their origin from the bulb is 



better seen in Fig. 24, where a portion of the two branches of the 

 tentacular apparatus, extending along this groove, is represented. 

 It is exactly as if we had the tentacles of a Pleurobrachia, instead of 

 swimming and floating freely about, protected by a kind of cover, and 

 thus pressed towards the spherosome, and prevented from moving 

 freely about. The whole spherosome is covered with minute spots, 

 clusters of lasso cells scattered irregularly over the surface. (See 

 Fig. 23.) 



From what we know of the amount of water which enters into the 

 composition of Acalephs, and when we remember that not more than one 

 half of one per cent, is animal matter, it seems strange that anything 

 like a parasite should be found upon these Acalephs, and stranger still 

 that this parasite should be able to find enough to live upon in such 

 a delicate animal. As early as 1835 Sars had observed a species of 

 intestinal worm (Scolex acalepharum) upon a large species of Mnemia 

 (M. norvegica Sars), ten and even twelve specimens being found at- 



Fig. 24. A part of the tentacular apparatus, near the opening of the actinostome, to show the 

 mode in which the branches of the tentacle extend, under cover of a lappet, towards the abactinal 

 extremity. 



