78 MARINE ANIMALS OF MASSACHUSETTS BA?. 



Besides these locomotive members, the community contains three 

 kinds of Hydras arising as buds from the primitive Hydra below 

 the swimming bells, the latter remaining always nearest the oil 

 bubble at the top, while the first Hydra, the founder of the com- 

 munity, in proportion as the new individuals are added, is gradu- 

 ally pushed downward, and remains always at the end of the 

 string, the stem of which is formed by the elongated neck of the 

 primitive Hydra. All the three sets of Hydrae have certain fea- 

 tures in common, while they have other distinguishing character- 

 istics marking them as distinct individuals. They are all accom- 

 panied by triangular shields (Fig. Ill), arising with them at the 

 Fig ul same point on the parent stem, and all 



are furnished with tentacles hanging 

 down from the summit of the Hydra at 

 the side opposite the shield. These facts 

 are important to remember, since we 

 shall presently perceive, upon analyzing 

 their parts, that these Hydrae have a close 

 homology to the Hybocodon. The ten- 

 tacles differ in structure as well as in 

 number for each kind of Hydra. Hav 

 ing shown in what characters they 

 agree, let us now take each set individ- 

 ually, and see what differences they 

 present. 



In the first set which we will exam- 

 ine the Hydra is open-mouthed. Like 

 the original Hydra, it is only a digestive 

 tube, similar in all respects to the proboscis of a Medusa-disk. 

 Its only function is that of feeding, and it shows a laudable fidel- 

 ity to its calling, being very constantly and earnestly engaged in 

 the work. Let us add, however, that in this instance the occu- 

 pation is not a wholly selfish one, since the cavity of every Hydra 

 communicates with that of the stem, and the food taken in at 

 these ever-gaping mouths, is at once circulated through all parts 

 of the community, with the exception of the oil bubble, from 

 which it is excluded by the transverse partition dividing it from 



Fig. 111. Cluster of Medusae with tentacles having pendant knobs. 



