54 MARINE ANIMALS OF MASSACHUSETTS BAY. 



state, forming slight projections at the base of the two other 

 tubes. Fig. 66 gives a view of the animal from below, at this 

 stage of its growth, while Fig. 65 shows it in profile. It will be 

 seen by the latter how very spherical is the outline of the disk at 

 this period, while the proboscis, in which are placed the mouth 

 and digestive cavity, is quite long, and hangs down considerably 

 below the lower surface of the disk. As the animal advances in 

 age the disk loses its spherical outline, and becomes much flat- 

 tened, as may be seen in Fig. 67. It may be well to introduce 



here some explanation of the law ac- 



Fig. 67. 



cording to which the different sets of 

 tentacles follow each other in succes- 

 sive cycles of growth, since it is a law 

 of almost universal application in Jelly- 

 fishes and Polyps ; and, owing to the 

 smaller number and simpler arrange- 

 ment of the tentacles in Oceania, it may be more easily analyzed 

 in them than in many others, where the number and complication 

 of the different sets of tentacles make it very difficult to trace 



their relation to 

 each other dur- 

 ing their successive 

 growth. We have 

 seen that the Oce- 

 ania begins life 

 with only two ten- 

 tacles. These form 

 the first set, and 

 are marked with 

 the number 1 in 

 the subjoined dia- 

 gram, which gives 

 the plan of all the 

 different sets in 

 their regular order. The second set, marked 2, consists also of 

 two, which are developed at equal distances between the first two, 

 i. e. at right angles with them. The third set, however, marked 3, 



Fig. 67. Young Oceania, older than Fig. 65 ; magnified. 



