24 INTKODUCTION TO CLASSIFICATION. 



the Adinozoa does not lie free in the interior of the body, but is 

 connected to the sides of it by means of membranous partitions, 

 the so-called " mesenteries " (/), which pass radially from the 

 stomach to the side walls of the body, and so divide the " peri- 

 Fig. 7. 



Fig. 7. Perpendicular section of Actinia holsatka (after Frey and Leuckavt); a, mouth ; 

 b, alimentary cavity ; c, common cavity ; d, intermesenteric chambers ; e, cord con- 

 taining thread-cells at the edge ; /, the mesentery ; g, reproductive organ ; h, tentacle. 



visceral cavity " into a number of chambers, which communicate 

 with the bases of the tentacles. In the whole of the Hydrozoa 

 the reproductive organs were attached to the exterior of the 

 body, and projected from it. In the whole of the Adinozoa, 

 on the other hand, the reproductive organs (of which both sexes 

 are frequently combined in the same individual) are internal, 

 inasmuch as they are situated in the substance of the mesen- 

 teries (g). 



These are the universal and distinctive characters of the 

 Adinozoa. That some are simple and some are compound 

 organisms ; that some are fixed and some free swimmers ; that 

 many are soft, while a great number are provided with very 

 dense skeletons; that some possess a rudimentary nervous 

 system, while the majority have as yet afforded no trace of any 

 such structure are secondary circumstances in no way affecting 

 the problem before us, which is, to find a diagnostic definition 

 of the group. 



