214 HYDROID^. Pakt IV. 



tractions, owing to the difference of structure of its different parts; the lower 

 extremity, which is cajjable of the greatest dilatation, differing somewhat from the 

 main body, and this again from the upper portion of the tube, which enlarges 

 into a central cavity. This tube, or proboscis, when contracted, does not extend 

 beyond half the depth of the main cavity of the body. It is even, at times, 

 shortened beyond this limit. In its utmost state of contraction, the lower opening 

 is rather widened, and the proboscis may then be compared, in some degree, to 

 the mouth of other Medusas, though its margin is not split into lobes. When 

 relaxed, it either hangs straight downward or forms undulations in its course, and 

 hangs then, generally, not only to the lower margin of the main cavity, but more 

 or less beyond it. When greatly elongated, it may hang between the tentacles 

 for three times the length of the body itself The upper part of the tube, in 

 the centre, is always thinner than the middle and lower portions. To this middle 

 part the eggs are attached. From the central cavity, into which the proboscis 

 empties, arise, at right angles with each other, four chymiferous tubes, com- 

 municating freely with the central cavity, as well as with the cavity of the pro- 

 bo.scis. These four tubes, following the inner surface of the gelatinous disk, extend 

 to its lower margin, where they are united with each other by a circular tube, 

 of the same appearance and the same diameter, forming a continuous canal around 

 the lower jjart of the disk. This circular tube communicates as freely with the 

 vertical radiating tubes, as these communicate with the central cavity ; so that 

 digested materials, and the water in which the food is dissolved, and with which 

 it is mixed in greater or smaller quantity, circulate freely, to and fro, in all 

 the parts of this apparatus. It is astonishing how quickly an animal, swallowed 

 by this little Medusa, is dissolved, and its particles circulated throughout the system. 

 The digestion takes place above the mouth, which shuts over the food, or sim23ly 

 stretches upon the surface of the animal upon which it feeds, sucking its juices, 

 and immediately after dropping its dead carcass. In that way our Sarsia swallows 

 very quickly large numbers of small Medusae, and especially other species of Hydroid 

 Medusae and the young of Aurelia fiavidula, and also other soft animals and small 

 Crustacea ; I have, however, never seen it swallowing the hard parts of any of 

 the latter, but only sucking their juices. 



The liquid food thus secured is moved on, through the proboscis, in jerks, to 

 and fro, under the contractions of the tubular proboscis. It takes, however, some 

 time for the contents of the stomach or probo.scis to pass entirely into the central 

 digestive cavity, into which they are finally i^ressed, mingled with more or less 

 water; as a constant process of regurgitation is going on, so that particles which 

 were at one time near the upper end of the proboscis, are now and then suddenly 

 pressed back into the lower end of that organ, the contractions of the mouth 



