DEMJLOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 717 



were wanting in the other two. Hence I cannot but agree with him' in regarding them 

 as non-essential parts of the skeleton, belonging to its jicrisomafic and not to its radial 

 system (§ 84). 



40. Arms. — Each of the ten Arms formed by the bifurcation of the Eays, is composed 

 of a long succession of segments, gradually diminishing in diameter from its base to its 

 termination. The number of these segments appears to me to have no definite limit; 

 for in every case in which the arm has presented the aspect of completeness, I have 

 found its termination exhibiting the same indications of continued growth as are mani- 

 fested by it when obviously immature (§ 67). It is remarkable that although the forma 

 and proportions of the segments vary widely in different parts of the Arm, — tlieir length 

 being four or five times their diameter near its extremity (Plate XXXVIII. fig. 4), 

 less than twice their diameter about its middle (Plate XXX^^II. fig. 1), and less than half 

 their diameter near its base (Plate XXXVII. fig. 3), — this diversity almost entirely 

 results from the progressive increase in diameter which shows itself in the segments as 

 we pass from the extremity towards the base ; the absolute length of the segments being 

 neai-ly the same throughout. The average length of each segment is rather less than 

 •03 inch ; and thus in an arm of which the total length is 4 inches, we ha^'e about 

 140 segments. The general plan of their conformation is everywhere the same, each 

 segment being fundamentally a cylindrical rod, perforated by an Axial Canal ; and the 

 departui'es from this form have reference chiefly to the attachment of the muscles and 

 ligaments at the articular surfaces (Plate XXXVIII. figs. 2, 4). But the diversity in 

 the proportions of the segments in the different parts of the Arms gives a great variety 

 to their general aspect ; and it will hence be desirable to describe separately the Basal, 

 the Middle, and the Terminal portions of the Arms. 



41. Basal Portion. — The diameter of the Arms near their base in a full-grown speci- 

 men is about -07 inch ; and this is maintained with little diminution for nearly a quarter 

 of their length. When we look at them from the dorsal side (Plate XXXVII. fig. 3), 

 we see that the inner margin (that is, the one which looks towards the otlier arm of the 

 same Ray) of ih.Q\vjj7'st segments is so much shorter than their outer that, whilst the two 

 articular facets of the Third Radial incline towards each other at an angle of about 80°, 

 that angle is widened-out between the distal articular faces of the two first Brachials that 

 rest upon it to about 130° ; and that by a similar difference in the length of the inner 

 and outer margins of the second Brachials, their distal articular faces are brought nearly 

 into the same plane. Generally speaking, a like inequality shows itself between the 

 inner and the outer margins of the succeeding segments, but in an alternating manner ; 

 so that their dorsal faces have the form of a succession of triangles, the apices of which 

 point altei-nately to one side and the other, their vertical angles being 

 about 40°. But there are many departures from the regularity of this 

 arrangement ; the most frequent being that which is produced by the 

 close union of two segments having flattened articular surfaces whose plane is directly 



' " On the Embiyogcny of Antedon rosaceus," Pliilosophical Transactions, 1805, p. 541). 



