274 CTENOPHORiE. Part II. 



Tliougli our Idjia is slow in its locomotion, its movements are particularly 

 graceful. Its most common attitude is horizontal ; tliat is to say, the main or 

 vertical axis is generally maintained in a horizontal position, or rather shghtly 

 inclined, so that the actinal pole, -which moves forward, stands higher than the 

 abactinal, which is turned backward. In this respect Idyia coincides with Pleuro- 

 brachia, which also moves with the actinal pole forward ; but, in its most common 

 attitude, Pleurobrachia stands with its vertical diameter upright. Again : Idyia, while 

 moving horizontally, keeps almost uniformly vipon its broad side, and may readily 

 raise its actinal end, with the mouth gaping and ready to seize its prey, as in 

 PI. I. Fiff. 8, the flatness of the body facilitating the changes of its attitude. Occa- 

 sionally it turns, Avith one or the other side rising ; but I have never seen Idyia 

 revolving upon itself around its vertical axis, as Pleurobrachia constantly does, and 

 only now and then does it make somersaults, turning upon its vertical axis in the 

 direction of the broader plane. It should further hQ remarked, that the older the 

 specimens grow, and the larger they are, the more sluggish become their movements. 

 Tlie young are far more active, and the smallest of them are comparatively quick ; 

 but the motion is always a gliding one, long continued in the same direction, with 

 the body stretched to its full length. Only now and then a j^owerful contraction 

 may be noticed, during which the animal reduces its greatest diameter by at least 

 one third, and, as the spherosome is extremely flexible, it then assumes very varied 

 forms, according to the condition of the digestive cavity. When this is empty, the 

 actinal end may even be turned inside during such contractions {Fic/s. 5 and 5") ; 

 the vertical diameter is then reduced by about one half, and the outline becomes 

 almost circular {Fig. 5"). When the contraction is one-sided, the body curves in 

 various directions. The mouth also may spread or contract, so as to assume the 

 most different outlines : at times gaping widely {Fig. 1), at other times contracting 

 in the centre with the opposite ends wide open {Fig. 9), or bending sideways {Fig. 2"), 

 or closing up in a straight line {Fig. 4), or even shutting by the inversion of its 

 edges {Fig. 1a). When the digestive cavity is gorged with food {Fig. 10), the body 

 may be distended in any direction and assume the most irregular shapes. 



Our Idyia is very voracious, and feeds chiefly on other Ctenophora). Whenever 

 I have kept Bolinas and Pleurobracliias in the same vessel with it, they rapidly 

 disappeared, being generally swallowed entire. In the attempt to seize upon its 

 prey, Idyia readily changes the direction of its motion, but always keeps the mouth 

 forward toward its prey, gaping widely as in Fig. 1, and Avhen close upon it turning 

 the edges of the mouth still further outward. A smaU Pleurobrachia or a small 

 Bolina would frequently pass into its -ndde digestive sac without any other eflbrt 

 on the part of the Idyia than that of shutting its mouth. But when its prey 

 is larger, perhaps nearly of its own size, our Idyia may be seen distending its 



