exxvi Introduction. 



take place either in tlie way of parthenogenesis, as in Rotifera and 

 Ascaris nigrovenosa in its ento-parasitie stage ; or in that of meta- 

 genesis from a part of a protozooid, which is not differentiated as a 

 sexual gland. Metagenesis is observable both in the highest and 

 in the lowest of the Vermes. 



In some instances, at the time of the setting free of the deii- 

 terozooid produced by gemmation, both protozooid and deutero- 

 zooid may be in the asexual condition, as is ordinarily the case 

 with Nais and Chaetogaster amongst Annulata, and with Micro- 

 stomeae amongst Turhellaria. In the cases of certain other Tur- 

 hellaria {Strongylostoma and Catenula), no other than this simple 

 metagenetic form of reproduction has been, as yet, observed. When 

 this form of reproduction alternates as in the other Turhellaria, and 

 in the Annulata just mentioned, with sexual reproduction, we have 

 a series of phaenomena before us which has been spoken of as 

 ' Digenesis with Heterogony.^ In some cases, as occasionally in 

 Microstomeae, and in the Annelidan Syllidea and Protula, a sexual 

 protozooid has been observed to give origin by gemmation to a 

 sexual deuterozooid, furnishing thus an example of digenesis with 

 contemporaneous heterogony. In some rare cases, the sexual 

 organs of the protozooid have been observed to be of one, and those 

 of the deuterozooid of the other sex. In other cases, the protozooid 

 is always asexual, when it is known as a ' nurse j'' whilst the 

 deuterozooids it gives origin to attain the sexual condition, either 

 whilst still attached to the parent organism, as in the Taeniadae 

 amongst Platyelminthes, and Autolytus amongst Annulata ; or 

 subsequently to detachment from it, as in Trematodes. These forms 

 of reproduction may be spoken of as cases of ' digenesis with alter- 

 nation of generations,' inasmuch as the asexual forms or stages of 

 metamoi-phosis interposed between the products of sexual congress 

 and another set of sexually perfect individuals may be regarded 

 as sufficiently distinct and independent to merit the title of ' gene- 

 rations.'' 



The relations held to the adult forms by the provisional organs of 

 the larvae of certain Turbellarian worms (Pilidium), and also of 

 certain Gephyrea (Actmotrocha), bear a considerable resemblance to 

 those observable in the larvae of Echinoderms. The Oligochaetous 

 and the Discophorous Annulata do not go through any metamor- 

 phosis. In worms of parasitic habits, even where, as in Nematoids, 



