128 THE VOYAGE OF H.M.S. CHALLENGEE. 



arclienteron was not the only, nor perhaps the most important part of the organism to 

 act upon. 



Still more different are they from those advocated by Perrier ^ and Cattaneo,' who 

 have adhered to and extended the idea already held by others, but by them most actively 

 defended, " that the metamery of Arthropods, Vertebrates, and a great many Vermes, 

 has originated out of the multiplication by transverse fission of very simple primitive 

 worms which were not metamerous. The products of this transverse fission remaining 

 connected together have then formed a chain of individuals, or a linear colony ; later on 

 the unity of the chain has become more definitely established, the single individuals at 

 the same time becoming difi"erent both in form and in function, and the foremost 

 individual thus becoming the head of the series. Each segment (metamere) thus repre- 

 sents a reduced individual ; a metameric (segmented) animal is the result of the more or 

 less complete fusion of single individuals into an individual of higher order." 



Emery, from whose paper ^ I have translated the foregoing sentence, has very success- 

 fully combated these propositions. This author, however, adheres to Lang's views in 

 ascribing to the archenteric pouches, the " gemmation " as Emery calls it (loc. cit., p. 18) of 

 the intestine, the most important and initial significance for the first origin of metamery, 

 " the sexual glands and excretory canals being in relation to the intestinal diverticula," 

 and following the lead. I have above explained why I cannot adhere to this argumen- 

 tation, which brings the ccelome and the sacculated intestine too strongly into the fore- 

 ground, and why I rather suppose incipient metamery to have been antecedent to either 

 of these {e.g., Carinella). On the other hand, many views contained in Emery's im- 

 portant paper coincide with my own. Thus he writes {loc. cit., p. 11), speaking of that 

 interesting marine Triclade, Gunda segmentata : — 



" The metamery of Gunda is thus manifestly the consequence not of the ' symbiotic ' 

 fusion of a colony of equivalent ' parts ' (meridi), but of the ' autobiotic ' difi"erentiation 

 and perfectioning of one ' part ' (meride);" and further on (p. 15) : — " When I consider 

 the facility with which certain worms break into one or more pieces even spontaneously, 

 it appears to me that this capacity for rupture may well have been the origin of the 

 reproductive purpose of transverse scission in similar elongated organisms. The rupture, 

 in the first instance accidental, could have contributed to the more rapid multiplication of 

 the organism, being followed by the regeneration of the parts that were deficient in the 

 separate fragments. This process of rupture might further have been so perfected that the 

 spot best adapted for rupture, with a view to the best condition of the fragments, was 

 prepared in advance. In the more perfect evolutional phases of the process, which are at 

 the same time those that have till now been more carefully investigated, the new head is 

 formed anteriorly to the rupture, or at least its essential parts are pre-established." 



1 E. Perrier, Les colonies animales, Paris, 1881. 



- G. Cattaneo, Lt- colonie lineari e la morfologia dei Mollusclii, Milaiio, 1883. 



•* C. Emery, Colonie liueare e metameria, Napoli, 1883. 



