80 MARINE ANIMALS OF MASSACHUSETTS BAY. 



is closed ; they have, therefore, no share in feeding the com- 

 munity, but receive their nourishment from the cavity of the stem 

 into which they open. They differ also from the others in having 

 a single tentacle instead of a cluster^ and on this tentacle the lasso- 

 cells are scattered at uneven distances (Fig. 114). 

 The special function of these closed Hydras is yet to 

 be explained ; they have oil bubbles at their upper 

 end (see Fig. Ill, the top Hydra), and though we 

 have never seen them drop off, it seems natural to 

 suppose that they do separate from the parent stock, 

 and found new communities similar to those from 

 which they arise. 



The intricate story of this singular compound ex- 

 istence does not end here. There is still another set 

 of individuals whose share in maintaining the life of 

 the community is by no means the least important. 

 Little bunches of buds, of a different character from 

 any described above, may be seen at certain distances 

 along the lower part of the stem. These are the reproductive 

 individuals. They are clusters of imperfect sexual Medusae, re- 

 sembling the rudimentary Medusae of Tubularia (Fig. 99), which 

 are never freed from the parent stem, but discharge theh* contents 

 at the breeding season. Like many other compound Hydroids, 

 the sexes are never combined, in one of these commimities ; they 

 are always either male or female, and as those with female buds 

 have not yet been observed, we can only judge by inference of 

 their probable character. From what is already known, how- 

 ever, of Hydroid communities of a like description, we suppose 

 that the process of reproduction must be the same in these, and 

 that the female stocks of Nanomia give birth to small Jelly- 

 fishes, the eggs of which become oil bubbles, similar to that with 

 which our little community began. (Fig. 108.) 



By the time all these individuals have been added along the 

 length of the stem, the stem itself has grown to be about three 

 inches long (Fig. 115), though the tentacles hanging from the 

 various members of the community give to the whole an appear- 

 ance of much greater length. The motion of this little string of 



Fig. 11-t. Medusa with a simple thread-like tentacle. 



