of the Bay of Naples. 107 



which are in process of growth into ova, while the second 

 (viteHigene) merely furnishes the nutritive material necessary 

 for development. This family of the Ductifera, to which by 

 far the greater number of genera belong, possesses a special 

 duct for the reproductive materials opening in the back. The 

 nervous system presents a further decided distinction between 

 the two divisions. The Ductifera always* present two lateral 

 and one or two dorsal tufts of tactile seta3, while in the Philo- 

 dinaese only the latter occur. The marine Rotatoria also, upon 

 which, however, we have no recent investigations, enter 

 without violence into these natural groups, so far as I can 

 judge from the examination of certain genera {Brachionus^ 

 Pterodina, Cohirus, Synchceta, Furcularia, and Philodincem) , 

 as they only differ very slightly from their allies in the fresh 

 water. But the three genera parasitic on Nehalia differ so 

 much from all other Rotatoria that in opposition to these they 

 must be united into a third family. Undoubtedly, however, 

 the Seisonidte come nearer to the Philodinaste than to the Duc- 

 tifera. With the former they share the elongate worm-like 

 form, and the faculty of pushing the foremost and hindmost 

 apparent segments into one another in telescope fashion. 

 The adhesive organ of the tail is formed in both sections by 

 a considerable number of glands, while the Ductifera only 

 possess two, or have lost them altogether ; and further, in tlie 

 nervous system, the absence of the lateral feelers is common to 

 them. As the primitive construction of the rotatory apparatus 

 of the Philodiua^as renders it probable that this family has 

 retained comparatively many of the characteristic peculiarities 

 of the original form of the entire class, we may also assume 

 that the fSeisonidas branched off comparatively early from the 

 genealogical tree of the Rotatoria. A further conclusion 

 arising from this (which, however, like all such phylogenetic 

 speculations, must, of course, be taken with the necessary 

 reserve) would tlien be that the female sexual organs of the 

 Rotatoria were originally paired, and consequently the un- 

 paired sexual apparatus of most members of the class is to be 

 regarded as something acquired secondarily. This view is 

 supported by the structure of the sexual organs in Plerodina 

 patina^ Ehr,, a species whose genus (as I have previously 

 indicated) is one of the few Ductifera in which several primi- 

 tive peculiarities of organization have been preserved j the 

 Pterodince have the simple two-rowed rotatory apparatus of the 

 Philodin^ai, their water-vascular system has not yet developed 

 any contractile vesicle, and the long tail terminates with the 

 same tuft of cilia which characterizes the young forms of the 

 * To this Conochilus is au exceptiou. 



