238 



A COMPARISON OF 



belies the relationship which its internal organization reveals. 

 In its younger stages (fig. 25, A, B) its exterior form possesses 

 the characteristic obliquity of the type to which it unquestion- 

 ably belongs. 



In the Hydra, on the contrary, we find altogether different ten- 

 dencies. "What are the peculiarities of the type to which it be- 

 longs I have already (p. 177) described, when speaking of the 

 organization of Zoophytes ; but I wish at this point to say a few 

 words in explanation of those nearly related Hydroids which 

 form the connecting link between it and the more complicated 

 forms. The first one that I shall take up is interesting because 

 it not only illustrates this transition, but also displays a bilateral 



character in a most unquestionable light. It 

 is a Hydroid by the name of Tubularia, (fig. 

 hd 136,) and is common on the whole North At- 

 lantic coast, both in 

 Europe and America. 

 The cylindrical stem 

 (s to s 1 ) supports a 

 distinct head, (hd,) 

 which at the base 

 bears a wreath of 

 twenty to twenty-five 

 tentacles, (,) and pro- 

 jects into a broad, cy- 

 lindrical proboscis, (/?,) whose end is pierced 

 by the mouth and covered by several rows 

 of short tentacles (2 1 , ft). Upon cutting 

 across the stem, we find that it does not 

 embrace a single cavity like that of the 

 Fig. 136. Hydra, but that its centre (fig. 137, c) is a 



Fig. 136. Tubularia indivisa. Lin. Natural size. Marine. From Boston 

 Harbor, s to si, the stem ; hd, the head ; t, the posterior tentacles, or corona ; 

 /I, ft, the tentacles of the proboscis (p). Original. 



Fig. 137. An actual transverse section of the upper part of the stem of fig. 



