104 CLASS III. 



to the motion of the colony, but sway gently to and fro with the 

 contractions of the stem. The Feelers are cylindrical or vermiform 

 structures having much resemblance to the polyps, but without an 

 external opening. In some cases they are in constant motion, 

 feeling about in all directions : in others they are more sluggish 

 and are loaded with the general nutrient fluid. The latter fact 

 would seem to suggest a respiratory function. Some writers have 

 considered them to be receptacles for the fluid forced from the 

 interior of the prehensile apparatus during its violent contractions : 

 and hence the name, sometimes given to them, of fluid-holders. 

 But it has been objected to this explanation, that the two sets of 

 organs are often at great distances from each other, and that their 

 alternate action has not been established. 



The composition of all the organs in the Siphonophors cor- 

 responds to that of the other acalephs. As in these, the specific 

 gravity of the mass differs little from that of sea-water. The shell 

 found in some families, (velella, porpita] is the thickened and hard- 

 ened wall of the air-sac 1 .] 



On the Nervous System of Acalephs, the observations are 

 hitherto imperfect. In the Medusae EHRENBERG observed, at the 

 base of each of the eight marginal corpuscles which he takes to be 

 eyes, a part which he considers to be a ganglion. Each of these 

 ganglia is double, or consists of two limbs that diverge towards the 

 marginal corpuscle. Besides these a row of ganglia lies near the 

 tentacles at the margin of the disc ; every ganglion divides into two 

 twigs, each of them for one of two adjoining tentacles: so that 

 every tentacle receives two nervous twigs coming from different 

 ganglia. This ring of ganglia round the margin is interrupted by 

 the larger double ganglia of the marginal corpuscles. Moreover, 

 EHRENBERG saw four groups of ganglia lying in the cavities for 

 the four genital organs and in connexion with the tentacles of these 

 cavities. EHRENBERG could not detect a nervous ring round the 

 mouth, the usual form of the nervous system in Eaclials 2 . Some 

 writers are of opinion that it by no means follows from these obser- 

 vations that the parts so described are really nerves. GRANT de- 

 scribed in Cydippe pileus a nervous ring with eight ganglia, each 



1 Corap. LEUCKART Zoologische Untersuchungen, s. 3 41 ; KOELLIKER in Zeit. f. 

 Wissmsch. Zool. iv. s. 306 315. 



2 EHRENBEBG Die Aka-lepken- &c. s. 25, 26. 



