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VOL. XXXV. 



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Fig. 19. — Thalassomet- 

 rim:; Charitometra. 



ond (axillary) post-radial joints, but nowhere else; in Tropiometra 

 Z 2 and Z, are disproportionately large and broad, and we find the 

 first and second (axillary) post-radial joints 

 similarly enlarged ; any ornamentation or cari- 

 nation of r L x and Z 2 is always duplicated on the 

 two preceding joints. 



Passing now to the Comatulida with more 

 than ten arms (excepting Comaster marice, G. 

 fimbriata, C. coppingeri, ('. borneensis, 0. 

 multiradiata, G. iowensis, G. sentosa, G. lineata, 

 and G. discoidea) (figs. 15, 16, 19, 20, 22. and 

 23) ; these multibrachiate forms are always ten 

 armed until of considerable size, when, by a 

 process of autotomy, the arm is cast off at the 

 synarthry (or syzygy) between the third and 

 fourth post-radial (first and second post-axillary) joints, or at the 

 syzygy between the fifth and sixth post-radial (third and fourth post- 

 axillary) joints, and from the 

 stump an axillary grows replacing 

 the cast-off arm by two or more. 

 This process of arm reduplication 

 by autotomy was described by 

 Minckert in 1905, but was in- 

 dependently discovered by the 

 present author through observa- 

 tions made on quite different 

 material before Minckert's paper 

 was consulted. In the Coma- 

 tulida, as is well known, the 

 various " division series " of the 

 arm or ray between the first 

 post-radial axillary (second post- 

 radial joint) and the free un- 

 divided arm are composed of either two or four joints. If of two, 

 they are united either by synarthry or syzygy; if of four, the third and 

 fourth are always united by syzygy, while the 

 first and second are almost always united by syn- 

 arthry, but occasionally are united -by syzygy; 

 the two pairs, the first and the second, and the 

 third and the fourth, are united by a straight 

 muscular articulation between the second and 

 third. No matter how many axillaries may 

 intervene between the radials and the free un- 

 divided arm, we are always able to recognize Z x and Z 2 as the first and 

 second joints beyond the last axillary; and when the division series 



Pig. 20. — Himerometrid.e ; Himerometra; 

 also, comasteridje ; phanogenia. 



FIG. 21.— COMASTERIDiE 

 COMATULA. 



