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ME. H. M. BEENAKD ON THE 



[Aug. 1894, 



trunk-segments, and its more membranous parapodia-like limbs, 

 must be assumed to lie in tbe direct line upwards from the original 

 annelidan ancestor towards the modern Crustacea. The trilobites 

 then must have branched off laterally from this line either once or 

 more than once, in times anterior to the primitive Apus, as forms 

 specialized for creeping under the protection of a hard imbricated 

 carapace. This carapace was obtained by the repetition, on the 

 trunk-segments, of the head-shield which, as we have already seen, 

 almost certainly existed as a structure sui generis in earlier forms, 

 and, somewhat modified, has been retained as such in the early 

 Crustacea proper (' Aspidophora'). 



Reading downwards, we should arrange the relationship as 

 follows : — 



A richly segmented annelidan ancestor, with the first segment bent round, 

 so that the labrum and mouth point backwards, in order that the parapodia 

 may function as mouth-parts ; projections due to this bending round occur at 

 the sides of the first, or flexed, segment. 



The second segment fuses with the first to form a head of two segments. 

 The lateral projections, secondarily specialized, are repeated on the second 

 segment as pleurae, which fuse with the lateral projections of the first segment. 



Three segments form the head-region, and two pairs of pleura? fuse with 

 the lateral projections to form a head-shield. 



Four segments form the head- Microdiscus and other trilobites 

 region, and their lateral projections which have only four segments in 

 form the head-shield. This head- the head, and in which the head- 

 shield is not repeated as pleurae along shield is repeated as pleurae along 

 the trunk-segments. the trunk-segments. 



Olenellus and other trilobites with 

 "five head-segments. These may either 

 be deduced from trilobites with four 

 head-segments, or have branched off 

 independently from the main stem. 

 The pleurae are repeated along the 

 trunk-segments for a creeping manner 

 of life. With various formulae 

 of the cephalic limbs. 



Five segments form the head-_ 

 region, their pleurae forming the" 

 head-shield, which is not repeated as 

 pleurae along the trunk-segments. 



Head-shield developing backwards 

 as a carapace. Apus. 



Modern Crustacea. 



Trilobites (e. g. Ogygia) with six 

 segments forming the cephalic region, 

 due probably to the association of 

 the powerful limbs l of the sixth 

 segment with the mouth-parts. 



Limulus. 



Eurypterids (with 

 secondary degenera- 

 tion of the pleura?). 



1 Compare the special development of the first trunk-limbs of Apus, and of 

 Calymene according to Walcott's restoration. 



