Chap. II. LIMITS OF THE CLASS. 39 



whieli is traversed along its lower surface by radiating tubes. It requires l)ut little 

 familiarity -with tlie Medusae 'to know that the marginal fringes vary greatl}' in 

 number, as well as in structure ; some being hollow, while others are solid. These 

 appendages are not even present in all Medusa^ ; for neither the Rhizostomata nor 

 the Cassiopeia nor the Cephea) have them. The central opening presents also 

 marked differences in its outward termination. In some it has a simple rim, while 

 in others, four or more prominent angles may extend outward and assume the 

 shape of A'ery complicated ajjpendages. But in no Medusa is the margin of the 

 central opening inverted into the digestive cavity. 



Not so with the Actiniae {Fig. <S) and the other Actinoid Polyps. Here the 

 walls of the l)ody, whether soft, or hardened by calcareous ^„ g 



deposits, enclose a wide cavity, which is divided by radiating - ' ^ 



partitions into a number of chambers, communicating freely 

 with the so-called tentacles or marginal fringes. The central 

 opening does not commmiicate directly with the main cavity 

 of the body, but leads into a distinct digestive sac, suspended actixia makgixata, LoSneur. 



. -11 (Contracted and tlie tentacles 



ni the mam cavity. It is as it the upper jiart of the hollow drawn in.) 



cylindrical body had been turned into the cavity l>elow, its 

 edge hanging free and open in that cavity, though capable 



of closing by contraction. We have here, "then, two distinct t::^:.J^Z^!^Z^ 

 types; but they are homologous in all their parts. The ':^^Z^!^;^Z^::::^ 

 outer wall of the Actinia corresponds to the gelatinous disk raLli^ plrtitionT-Tono of the 

 of the Medusa, only that the centre of its outer surtiice is IX^^^l^'-" 

 so constructed as to enable these animals to attach them- '^'i«'»- -■' digestive sac. 

 selves by it, while in the Medusa it is unilbrmly rounded off, and affords no point 

 of attachment. The marginal fringes of the Actinia correspond to the marginal 

 fringes of the Medusa, only that in the Medusa they communicate directly with the 

 marginal circular tube, and through this with the radiating tubes, while in Actinia 

 they open directly into the radiating chambers. The radiating tuljes of the Medusa 

 correspond, it is true, to the radiating chambers of the Actinia ; but in Actinia these 

 chamljers open freely for their whole length into the centre of the main cavity of 

 the body, while in the Medusa the radiating tubes are closed cylinders, opening 

 only at their inner end into the main cavity. The central opening leads, in both, 

 into the main cavity of the body; but in Medusa the margin of that opening is 

 turned outward, and may be prolonged into large appendages, between the inner 

 surfaces of which a cavity is formed leading into the main cavity, Avhile in the 

 Actinia the outer margin of the central opening is turned inward and extends to 

 a considerable length into the main cavity, so that the inner surface of the sac so 

 fonned corresponds to the outer surface of the wall of the main cavity ; and it is 





n Baseof theaniuial. — /> OfK-uing ol 

 the tlii;estive sac leading into the main 

 cavity of the body. —cc OpeniDg lead- 

 ing from one radiating partition into 



