DESCRIPTION OF AN ABNORMAL LOBSTER CIIELIPED. 263 



tions Indicated by the dotted outlines in the same figure. Refer- 

 ring now to Fig. 6, which represents (by the unbroken outlines) 

 the actual positions of the claws, we find that whereas the primary 

 claw has rotated in the expected direction — though only about 

 45° instead of 80° — the secondary claws appear each to have 

 rotated towards their morphological dorsal sides about 35° from 

 the planes prescribed in the theoretical diagram (Fig. 7, full 

 outlines). These relations are perhaps to be accounted for as 

 follows : 



While there is some torsion in the base of a normal cheliped, 

 this is compensated for by the rotation of the appendage for- 

 ward, as a result of which the axis of articulation at the base 

 of the propodite, which is morphologically dorso-ventral, is es- 

 sentially vertical, as it is in the succeeding pereiopods. Hence 

 the position of the great claw is for the most part due to a torsion 

 in the propodite itself. In our abnormal specimen, the axis of 

 articulation of the propodite to the carpopodite lies also, as we 

 have seen, practically in a vertical plane; but, apparently as a 

 result of the mechanical hindrance of the secondary growth from 

 its base, the primary propodite has been able to turn only 45° 

 instead of 80° from that plane. 



The positions assumed by the secondary claws are a little more 

 difficult of explanation. Since, however, they have a tendency 

 to rotate in opposite directions, the plane {M') midway between 

 them should remain constant in position whatever the amount 

 of torsion, as shown in Figs. 6 and 7, and we may first consider 

 the relation of this plane to that of the primary claw. By ref- 

 erence to Fig. 7 we see that this plane, in the case of a double 

 extra process arising from the postero-ventral surface of an appen- 

 dage, should make an angle of 45° with the dorso-ventral plane 

 {d.-v.) . A glance at Fig. 6 will now show that the plane M' there 

 makes the same angle (45°) with the primary dorso-ventral plane 

 of the abnormal specimen. As a result, however, of the torsion 

 distad of ths point of divergence, the three claws no longer lie 

 in the same plane {pi., Fig. 7) which passes through the point 

 of origin of the extra parts, but a line connecting their centers 

 has to curve as shown by pV . in Fig. 6. 



We have already noted that the planes of the vsecondary 



