CIRCULATION, NUTRITION, AND RESPIRATION. 1:31 



This may be well seen in the stems of Tu/jularia indivisa, where the contained fluid is distriljuted 

 in numerous parallel streams ascending through some of the channels which are excavated in the 

 endoderm of the stem of this hydroid, and descending through others. The ascending streams 

 become confluent at the summit of the stem, and from this point the descending streams issue. 

 Every now and then the direction of the streams is reversed, the fluid flowing down the channels 

 which just before carried ascending streams, and flowing up in those through which the streams 

 had previously descended. Here and there the longitudinal channels conununicatc with one 

 another by transverse branches, and thnmgh these branches the fluid may be seen flowing from 

 one longitudinal cliannel to another. Occasionally the fluid will remain for a short period at rest 

 in one of the channels, while it continues to flow in its ascending and descending streams in the 

 others. The somatic currents of Tiibularia, however, are by no means always easily seen, and it 

 is only now and then that a specimen occurs, which, from the transparency of its tissues, and 

 the abundance of floating corpuscles in the somatic fluid, presents conditions favorable for 

 observation. 



In Cori/morpha mduns, the structm'e of whose stem resembles that of Tuhidaria indimm, 

 similar definite currents may be witnessed ; and in Anfennuluria aidennina, whose cceuosarc, as 

 already mentioned, is composed of numerous tubes within a common perisarc, currents of an 

 entirely similar kind may be seen ascending in some of these tubes, descending in others, passing 

 over from one to another by intercommunicating branches, and again resting in others, but ready 

 to start ofi' in a fresh stream upwards or downwards while we continue to watch. 



It is rare, however, to find the currents of the somatic fluid presenting the regularity and 

 definiteness which characterise them in the instances just mentioned. In those far more nume- 

 rous cases in which the somatic cavity is simple instead of being composed of separate and dis- 

 tinct channels, the contained fluid may occasionally be seen in a single broad stream flowing 

 through the axis of the stem or branch, then coming to a state of rest and after a short peiiod 

 of repose, starting off' anew in a reversed direction. Most usually, however, the movement in 

 such cases, instead of being in the form of a definite current, consists in an irregular commotion 

 in which the floating corpuscles are whirled about in all directions. Within more limited cavities, 

 such as that of the spadix in the sporosac and of young buds in process of development, the 

 motion of the fluid is generally in circular streams, which may be seen coursing round the walls 

 of the cavity and returning into themselves. These streams in the developing bud usually pre- 

 sent great activity. A similar disposition of the streams in circular currents may be witnessed 

 in the cavity of the hydranth ; but the motion here ought probably to be referred to digestion 

 rather than to circulation, as it does not seem to have any proper distributional office, being most 

 likely specially connected with the preparation of the aliment. 



In the gastrovascular canals of the medusa, the currents are very distinct, the transparency 

 of the surrounding tissues rendering it easy to observe them. In all these canals, whether radiat- 

 ing or circular, they constitute simple streams, which in some of the radiating canals may be seen 

 flowing from the proximal towards the distal end, and in others from the distal towards the proxi- 

 mal, and, as it would seem, reversing themselves every now and then in all these tubes. In the 



quently described by Lister, who also detected them in the stem of Tubuluria indivisa (' Phil. Trans.,' 

 183-4). Since then the currents in the somatic cavity of the Hyduuida have become familiar to every 

 observer of these animals. 



