564 



MERISTIC VARIATION. 



[part l 



*S6S. 



to below, in evidence that the total number of cases is considerable. 

 There are only two certain cases of double head (see Typosyllis, No. 868, 

 and Allolobophora, No. 873). 



POLYCH.ETA. 



Typosyllis variegata : individual having two small heads, as 

 shewn in Fig. 208. Heads of unequal size, that od the left having 4 

 segments behind the eyes, while that on the right had two. The 



869. 

 870. 



871. 

 872. 



Fig. 208. Typosyllis variegata, No. 868, having two small heads. 



(After Langerhans.) 



appearance suggested that the original head had been broken off and 

 that two new ones had grown in its place. Langerhans, P., Nova 

 Acta Ac. C. L. C, xlii. p. 102, PL 



Nereis pelagica : bifid posteriorly. Bell, F. Jeffrey, Proc. Zool. Soc., 1886, p. 3. 



Salmacina incrustans (Serpulida?) : posterior end double. [Two tails shewn in 

 figure as of equal length and in the same straight line, at right angles to the body. 

 The arrangement of the segmentation at the junction is not clearly shewn.] 

 Claparede, Mem. soc.phys. et dltist. nat. Geneve, xx. 1869 — 70, p. 177, PI. xxx. fig. 5 f. 



Proceraea tardigrada (Syllid*) : tail double; two specimens. In one of these 

 the tails were nearly equal, but one had no anal cirri. Andrews, E. A., Proc. U. S. 

 Nat. Mus., 1891; xiv. p. 283, and Amer. Nat., 1892, xxvi. p. 729, PI. xxi. 



Branchiomma sp. (Sabellidre) : two posterior ends, one being rudimentary. 

 Brunette, Trav. Stat. Zool. de Cette, 1888, p. 8 [quoted from Andrews, I. c] 



[With these conditions compare Syllis ramosa, a form found by the Challenger 

 in two localities, inhabiting a HexactineUid Sponge. The body of this creature con- 

 sisted of vast numbers of branches, about as thick as thread, passing off at right angles, 

 coiling upon each other and forming inextricable masses. In some specimens no 

 head was found, but a single head was afterwards discovered. It seemed likely that 

 large tracts of the body have no head, but there was no evidence to shew how many 

 heads occur in the colony. Many female buds were found, and a single complete 

 male. McIntosh, Chall. Rej)., xn. p. 198, PI. xxxi.] 



