118 UR. J. S'l^'EPHENSOiSr ON TUli MORPHOLUaY, CLASSIFICATION, 



and two in Victoria in Anstralia. It can be derived from 

 MsgasGolides by multiplication of setae, or from Dijyorocliceta hy 

 the breaking np of the nephridia. Both these genera, Megascolides 

 and B'qjorochceta, occur both in India and Victoria. It is at 

 least not improbable that Sjyenceriella has been evolved separately 

 in India and Australia, from one or other of these genern, 

 perhaps from the same, perhaps from a different one, in the two 

 parts of its range. 



It is to be noted also, that confining ourselves to that con- 

 siderable group of genera, of the Megascolecinse with a single 

 gizzard, which are distinguished by the characters of tliese three 

 systems, the logical end of any line of evolution is Megascolex. 

 The order in which the changes in the three systems have 

 occurred varies in the different lines; the nephridia may be the 

 first to undergo their characteristic evolution {Megascolides line), 

 or the seta3 {Dlporochmta line), or the prostates {Woodwardia line). 

 But as we follow out the lines other changes are added ; and if, 

 in any line, all three systems pass from the primitive to the 

 derived condition, we arrive at Megascolex, with perichjetine 

 setae, micronephridia, and racemose prostates. 



There seems therefore to be an a 2Jriori probability that groups 

 of worms possessing certain combinations of characters, that is 

 certain genera, may have arisen more than once, and that the 

 genera as we have them to-day are polyphyletic. 



But we are not dependent altogether on a 2^'>'iori reasoning. 

 The case to which Benham refei-s is the multiple origin of 

 Microdrilus from Notiodrilus. The two genera are distinguished 

 by the condition of the posterior male oi-gans — in Microscolex the 

 " raicroscolecine reduction " has taken place (this indeed is the 

 origin of the term). On Possession Island (one of the Crozet 

 group, some distai:ice south-east of Cape Colony) Michaelsen foinid 

 two species of worms, obviously very closely related, one with the 

 original condition of the male organs (i.e. a Notiodrilus), the 

 other with the microscolecine condition (i. e. a Microscolex) ; 

 the deduction is that the latter species has been evolved on the 

 island from the former. The two species agree in the peculiar 

 pigmentation, in the arrangement of setae, glandular modification 

 of the integument in the neighbourhood of the genital pores, in 

 the form of the penial setae, form of the pi-ostates, and size and 

 shape of the spermathecal diverticula. The Microscolex was 

 represented by numerous examples, and so was not an individual 

 chance vai'iation ; and Michaelsen is doubtless right in holding 

 (8, 9) that it has been evolved in this rem.ote region from the 

 jyotiodrilus which occurs there. Of course, other species of 

 Microscolex have evolved elsewhere, from other species (con- 

 ceivably from one species) of Notiodi'ilus ; and thus Microscolex 

 has arisen at least twice, from different ancestors and at difi"erent 

 times. 



