HYDROIDA WITH PROTOZOA. 239 



solid mass of cells, and its periphery is occupied by several lon- 

 gitudinally arranged tubes (ch, c/t 1 ). One (ch) of these tubes is 

 much larger than the others ; and if we trace it toward the pos- 

 terior end of the stem, it appears as the original one from which 

 the others have arisen right and left. It is, therefore, the domi- 

 nant feature of the stem, and constitutes the basis upon which 

 to estimate the relations of the regions about it. A line drawn 

 through it to the opposite side (through A, B) divides the stem 

 into symmetrical right and left parts ; that is to say, the stem is 

 bilateral, and its periphery is occupied by several laterally re- 

 peated tubes. 



The crown of tentacles (fig. 136, t) at the base of the head is 

 not, as it appears to be, a single row, but is composed of at least 

 three circles placed one before the other relatively ; but yet as the 

 feelers of each circle alternate with those in the others, all three 

 circles are enabled to overlap each other so closely as to produce 

 the appearance of a single one. If, however, their development 

 is watched during the earlier stages of growth, their separate 

 origin can be easily verified.* 



The manner in which the tentacles of the proboscis comport 

 themselves now and then illustrates, in a very apt way, their 



136. 12 diam. a, the parchmenty sheath ; b, the outer wall ; d, the inner wall ; 

 c, the solid core ; ch, ch 1 , the channels at the periphery of c. Original. 



* This diagram (fig. 138) will serve to illustrate the manner in which the ten- 

 tacles of the crown originate and change their position during the process of de- 

 velopment of the young. Tubularia is usually born with eight tentacles in the co- 

 rona. These may be represented by those in the centre (i, i, A, D) of the figure. 

 Next, eight more (ir, n, B, E) begin to develop farther back than those of the first 

 group, but as they grow they gradually press forward and fill the intervals be- 

 tween the latter (i. e. as if ft were to follow the direction indicated by the arrow). 

 Meanwhile a third group (in, in, F, C) of sixteen springs up still farther back 

 than the second, each tentacle (in) alternating with those of the first (i) and 

 second (n) groups, and in process of time they too push forward and occupy the 

 intervals between the members of the first and second groups (i. e. as if a and c 

 were to follow the directions indicated by the arrows). This is the way in which 

 the tentacles (fig. 136, i) of the corona of an adult head happen to appear to be 

 in a single circle. Upon close examination, however, even of a very old head, 

 one may detect a difference in the relative level of these tentacles. 



