DEVELOPMENT OF AJTTEDON (COMATIJLA, LAMK.) ROSACEUS. 725 



this joint, — analogous to that which in the Decapod Crustacea is well known to exist in 

 the basal extremity of the first phalanx. I am inclined to the belief, however, that the 

 comparative frequency of fracture at the syzygies depends rather on the greater brittle- 

 ness of the Arm at these points ; the syzygal segments being held together only by 

 sarcodic substance, instead of (as elsewhere) by ligaments and muscles. Certain it is 

 that the reproductive power, instead of being limited (as in the Crab and Lobster) to 

 one particular articulation, may be exerted at any point in the Arm, or even in the Eay. 

 Thus we see in fig. 9 that a Ray has been torn off at the junction of the First and 

 Second Eadials ; since not only the pair of Arms, but the Second and Third Radials 

 which bear them are so much smaller than the rest, as to be obviously products of the 

 reparative process. It is worthy of remark that the syzygies of these reproduced arms 

 occur at the regular intervals. Another case of the same kind, in which the new Ray and 

 Arms have attained a more advanced development, is shown in Plate XXXVIII. fig. 8, a. 

 In other specimens in my possession, the reparation has commenced from the bifurcation 

 of the Arms, from the articulation between the first and second Brachials, and fi-om the 

 articulation between the second and third ; and here, again, it is interesting to remark 

 that the first syzygy always occurs between the third and fourth segments, although there 

 is often some irregularity (on the side of excess) in the interval between the first and the 

 succeeding syzygies. — It is much more common, however, for only the terminal portion 

 of an Arm to have been lost ; and the new growth by which it is being replaced often 

 affords a peculiarly good illustration of the mode in which the development of fresh 

 segments is accomplished (§ 67). 



56. It is curious that in some cases the act of reparation should be attended with 

 Monstrosity ; and this may be on the side either of excess or of defect. Of the former 

 we have an example in Plate XXXVIII. fig. 8, B ; where we see that a fracture having 

 taken place at the articulation between the first and second Brachial segments, the new 

 Second Brachial, instead of conforming to the ordinary type, resembles the Third or 

 Axillaiy Radial, and gives support to two Arms, which have already attained a consider- 

 able development. A case of the latter is shown in the specimen represented in Plate 

 XXXVIII. fig. 7, where the normal bifurcation of the Ray has not taken place, the 

 Third or Axillary Radial being deficient, whilst the Second (which has about twice its 

 normal length) bears a single Arm of full size, with its syzygies regularly repeated. 

 Whether this monstrosity had its origin in fracture, however, may be doubted ; and the 

 like doubt may be entertained in regard to another case of " monstrosity by excess," 

 which has presented itself to me in an oral Pinnule ; a bifurcation like that of the Ray 

 . into a pair of Arms presenting itself at the second segment, so that two full-sized pinnules 

 take the place of the ordinaa-y single pinnule. 



