HOMOLOGIES. 141 



of a rugose prehensile organ. The ramose nature of the digestive caeca, which are all connected 

 with a central cavity, is but a modification of the pinnate organ in the Nemerteans, the pinnae in 

 certain of the latter being even slightly branched. Moreover, the microscopic structure of the 

 walls of this system (with perhaps the exception of the inner coating of cilia) is similar, and in 

 both cases appears to combine the biliary with the intestinal system proper. In the Planarians, 

 however, there is no anus, while such is present in all the Nemerteans. The system as a whole 

 shows a higher degree of advancement in the latter, the connecting links apparently occurring in 

 the Anopla, whose mouth opens behind the ganglia as in the Planarians. 



The nervous systems of the groups are also related. The cephalic ganglia are two in each, 

 but those of the Nemerteans are connected by two commissures, a dorsal and ventral, whereas in 

 the Planarians there is only a single large commissural band, which is homologous with the 

 ventral of the Nemerteans. The separation of the ganglia in the latter is more distinct than in the 

 former, though this does not necessarily imply a higher type ; indeed concentration of nervous 

 matter is generally considered to be so. The senses in the groups are somewhat similar ; over 

 the entire surface touch is as exquisite in the one as in the other ; the organs of vision consist of 

 two well-marked series in each, viz. those with and those without apparent lenses or capsules, so 

 that the pre-eminence in this respect is hard to adjudge, though I am inclined to give it to the 

 Nemerteans. There is some doubt about auditory corpuscles or otolites in either group, though 

 Grafe and Keferstein mention their occurrence in certain Nemerteans. I have never seen such. 

 The function of the special cephalic pits and neighbouring glands in the same animals is involved 

 in obscurity. They may represent the segmental organs of the higher Annelids, or, perhaps, with 

 greater probability, may be taken as the homologues of the water-vascular system. 



In the circulatory system the Nemerteans much excel the Planarians. In none of the latter 

 is there any circulation in distinct vessels, while in the former group all possess such, the vessels 

 being filled with a more or less corpuscular fluid. It is true that a water-vascular system is 

 described by O. Schmidt in certain freshwater Turbellaria, and that Max Schultze mentions a 

 system of canals in Thysanozoon and Tolycelis, but I agree with Professor Keferstein, after a 

 careful examination of spirit-preparations, in considering farther investigation necessary, and that 

 in the present state of our knowledge we cannot admit this in the ordinary Planarians. 



Considerable divergence occurs between them in regard to the organs of reproduction, the 

 Planarians being hermaphrodite, while almost all the Nemerteans have the sexes separate. In 

 regard to the complexity of the sexual system the former excel the latter, whose organs consist 

 simply of a series of sacs placed along the sides of the body — for the development of ova or sper- 

 matozoa. Moreover, where hermaphroditism appears, as in Borlasia hermapkroditica and B. 

 Kefersteinii, respectively described by Professor Keferstein and A. P. Marion, one part of the body 

 has its sacs filled with spermatozoa and another with ova, or the male and female elements occur 

 in the sacs without definite order as regards position, so that the type of structure remains 

 unaltered, and essentially different from the arrangement in the Planarians. 



The development of the young in the two groups has certain features in common, others at 

 variance. Thus the ova of the Planarians in some instances produce ciliated embryos that have at 

 birth more or less the form of the parent, — which form they retain throughout. The group Enopla 

 of the Nemerteans agrees with the former ; while in the division Anopla the young either emerge 

 from a ciliated covering inside the egg-capsule, or they are produced from the Pylidium4ovm in 

 the free state. Moreover, some of both great Nemertean groups are viviparous, the ova being 



