ANNULOSA: ANNELIDA. 269 



they are always specialised to act as efferent ducts for the re- 

 productive organs. 



The nervous system in the Errantia has its typical form, 

 consisting of a double gangliated ventral cord, two ganglia of 

 which are appropriated to each segment. The prae-oesopha- 

 geal, or cerebral, ganglia are of large size, and send filaments 

 to the ocelli and feelers. 



The sexes in the Errantia are in different individuals, and 

 reproduction is usually sexual, though in some cases gemma- 

 tion is known to occur. The process of gemmation is carried 

 on by a single segment, and so long as it continues, the bud- 

 ding individual remains sexually immature, though the young 

 thus produced develop generative organs. Thus, there is in 

 these cases a kind of alternation of generations, or rather an 

 alternation of generation and gemmation ; the oviparous indi- 

 viduals producing eggs from which the gemmiparous indi- 

 viduals are born ; these, in their turn, but by a non-sexual 

 process, producing the oviparous individuals. While the form 

 of gemmation just alluded to has long been known as not 

 uncommonly taking place among the Errant Ann elides, no 

 example of continuous gemmation has until lately been re- 

 cognised in any Annelide. Recently, however, Dr M'Intosh 

 has described a remarkable species of Syllis (S. ramosa), which 

 inhabits a Hexactinellid Sponge from the Philippines, and in 

 which the thread-like body is intricately branched, giving off 

 lateral offsets, and thus becoming a truly composite organism. 

 This singular form is further remarkable in the fact that no 

 traces of a head have hitherto been discovered, so that it is 

 probable that the entire branched organism possessed but a 

 single head. 



Not only does gemmation occur among the Errant Anne- 

 lides, but, in a few instances, fission has been noticed to take 

 place. Occasionally, also, the males and females differ from 

 one another, and both may differ from the sexless forms, when 

 these exist. Thus, Heteronereis is founded upon the sexless 

 forms of Nereis ; whilst the species of the genus Autolytus, 

 amongst the Syllidea, exhibit a still more remarkable poly- 

 morphism, the males and females being extremely dissimilar, 

 and there being in addition a third sexless form, which pro- 

 duces the sexual individuals by gemmation at its hinder 

 extremity. 



The embryo usually appears, on its liberation from the 

 ovum, as a free-swimming, ciliated body, possessing a mouth, 

 intestine, and anus. The cilia are primarily diffused, but 

 become aggregated so as to form a single median belt, or two 



