FISSIPAROUS SPECIES OF TUBICOLAR ANNELID 349- 



zooids occasionally observed arise. For when, by the fissive process, 

 the Nais is divided into an antferior and posterior zooid, the last 

 segment of the former greatly enlarges, becomes divided into segments,, 

 and the anterior of these becoming a head, a new zooid is formed 

 between the previously existing ones ; this process is repeated in what 

 was the penultimate, but is now the ultimate segment of the anterior 

 zooid : and, again, in the anti-penultimate, so that at least a long 

 string of zooids is formed, each of which, except the last, is produced 

 from a single segment. 



Fourthly, According to Frey and Leuckart, whose observations have 

 been confirmed by Krohn (Wieg. Archiv., 1852), Autolytiis prolifer 

 multiplies in a somewhat similar way, but instead of each new 

 interposed zooid being formed at the expense of a fresh segment of 

 the anterior zooid — it is produced by the metamorphosis of a bud,, 

 or rather of a mass of blastema the equivalent of a bud, developed 

 from the under extremity of the last segment of the anterior zooid. 



Supposing further observation to confirm the distinctness of all 

 these modes of prolification, they might be classified according to the 

 amount of the already formed parental organism which enters into- 

 the produced zooid. 



1. All the segments of the latter were segments of the former, the 

 new products being merely cephalic organs. 



2. None of the segments of the produced zooid belonged to the- 

 parent zooid, but the former is a metamorphosis of a whole segment 

 of the latter. 



3. None of the segments of the produced zooid belonged to the 

 parent zooid, and the former contains hardly any of the primitive 

 substance of the latter, being developed by germination from its last 

 segment. 



It is clear that the prolification of Protula DjsteriwUl come under 

 none of these categories ; but is a combination of the first and second 

 methods. The abdomen of the produced zooid is a mere fissive 

 product of the parent, but its thorax is the result of the metamorphosis, 

 of a single segment of the parent into many segments. 



Quatrefages endeavoured to show that the relation of the produced 

 zooids of Sj/Zis to the anterior zooid was that of an " alternation of 

 generation," the former alone developing sexual products. Krohn 

 has however proved that no such relation exists in this case ; but on 

 the other hand he brings forward good evidence to demonstrate that 

 the posterior zooids ol Autolytus prolifer really are generative zooids,. 

 and alone develop the reproductive elements. The male zooids in 

 this case are widely different from the gemmiparous zooid; so dif- 



