24 ME. P. H. CAEPENTEE ON THE GENUS ACTINOMETEA. 



of the free and undivided secondary or tertiary arms is of precisely the same nature as 

 the union of the first and second segments of the primary arms home hy the radial 

 axillary. In fact, it is not at all uncommon for one of the primary arms to remain 

 simple and the other to divide, as in PL II. fig. 9, c, d, e, which shows that the arms 

 taken in the strict sense of the word, cannot he regarded as commencing from any point 

 but the axillary radials. 



(§ 17) In practice, however, it is more convenient to regard the arms of Comatula as 

 beginning from the last bifurcation, i. e. from that axillary the two branches borne by 

 which do not further divide, but remain composed of a series of simple brachial seg- 

 ments (b l} b 2 ), &c. In the ten-armed Coma tit lee the brachials are, of course, borne directly 

 by the radial axillary. But in those forms, such as Act. mvltiradiata, in which the sub- 

 division of the ten primary arms is carried to a very great extent, it is most convenient 

 to regard as brachials only the segments of the ultimate branches borne by the last 

 axillaries, and to give special names to the segments composing the primary and 

 secondary arms ; for we have already seen that the number of the segments composing 

 these arms, i. c. between every two successive axillaries, varies in different species, and 

 it consequently becomes desirable to have some system of nomenclature by which 

 these differences can be briefly indicated. Under these circumstances, therefore, the 

 term distichals may be applied to the segments composing the ramified primary arms of 



e Articulate as well as of the Tessellate 1 Crinoids, but onlv on the distinct under- 

 standing that they are really arm-segments and do not enter into the formation of the 

 calyx, as in the Tessellata ; so that the name is purely a conventional one, employed for 

 greater convenience in the description of species. 



Supposing the secondary arms borne by the distichal axillaries to divide again, we 

 may consider them as composed of two, three, or four palmar 2 segments, of which the last 

 is a " palmar axillary " (figs. 10, 11, p.a.), and bears two tertiary arms. These may either 

 remain simple and composed of brachial segments, or they may continue to divide more 

 or less frequently. The latter case, however, is somewhat rare ; for if complete series of 

 distichals and palmars be developed on each radius, the total number of arms rises from 

 10 to 40 ; and there are not many Comatula? in which this number is exceeded. 



If we apply this nomenclature to the species of which mention has already been made, 

 we should describe Antedon Savignii with 20 arms, as characterized by the presence of 

 three distichals composing each primary arm and bearing the brachials directly, while 

 in Antedon palmata, with 30 or 40 arms, there are only two distichals, which are fol- 

 lowed by two palmars in the secondary arms. Act. polymorpha, again, normally has 

 three distichals and three palmars (PI. II. figs. 7, 10), while Act. multlrad'iata, Mull., has 



1 /. e. As understood by MM. do Koninck and Le Hon. 



- I have been accustomed for some years past to use the term " palmar " to designato the secondary arms of the 

 Crinoids. Professor Huxley, in whose lectures I first heard it used in this sense, informs me that he believes it to 

 have been so employed by Miillcr ; but I have searched in vain through Midler's works for any definition of the term. 



In his description of tho Tessellate Crinoids, however, he describes the plates which continue the series of inter- 

 radials and interaxillaries in a peripheral direction as " interpalmaria;" and as these partially correspond to the 

 intervals between the secondary arms, when such arc developed, the latter may perhaps be not incorrectly regarded as 

 composed of " palmar" segments. It will now be apparent why " interpalmar " is not a very suitable designation 

 for the strictly " interradial " areas on the disk of Comatula, as was remarked in section 15. 



