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



crmus. In the intervals between them the organic basis of the one segment is directly 

 continuous with that of the other, little or no fibrous tissue being interposed. 



The muscular fibres of Actinometra correspond very closely with those of Antedon, as 

 described by Dr. Carpenter l and Ludwig 2 ; their expanded terminations are simply 

 applied to the surfaces of the calcareous segments to which they are attached, not passing 

 into their substance as the ligamentous fibres do (PL III. fig. 7, b m s ), and there is no 

 trace either of a sarcolemma or of transverse striation. 



(ii.) The Dorsal Cirrlii. 



(§ 38) The Dorsal Cirrhi of Actinometra do not appear, so far, at least, as my observa- 

 tions have extended, to be developed over such a large surface of the centrodorsal piece 

 as is the casein A ntedon. In all the specimens which I have examined the cirrhi are 

 limited to its margin, while its central portion is entirely free from them and usually 

 slightly concave. There is generally only one row of these appendages ; but small and 

 rudimentary ones may occasionally be found interposed between the large and full-grown 

 ones at the extreme circumference of the plate, thus forming the commencement of a 

 second row. The number of cirrhi existing at any one time upon the plate-like centro- 

 dorsal piece of Act. polymorpha varies, I believe, between 15 and 20. Three or, in large 

 specimens (PL VI. figs. 1, 2), four are attached on each side of its more or less distinctly 

 pentagonal margin, while in var. 1 (PL VI. fig. 14), and in one specimen of the type 

 (fig. 7), the total number reached 25. In var. 2 (fig. 16) there are only 10; while 

 in vars. 3 (fig. 20) and 4 there is no evidence, in the single specimens which I have 

 examined, of the existence of more than three perfect cirrhi in the adult state, as there 

 are no sockets around the margins of the centrodorsal plate for the attachment of a 

 larger number ; though there may be minute openings here and there, which appear to 

 have corresponded with the central canals of lost cirrhi, the sockets of which have been 

 obliterated by a later calcareous deposit. 



It is not a little singular that the dorsal cirrhi of Act. polymorpha, like the centro- 

 dorsal piece which bears them, should exhibit such a very slight range of variation, not 

 only in size but also in number (the three varieties just mentioned of course excepted) ; 

 for in nearly every other part of the skeleton the range of variation is very great. In 

 Antedon rosacea the reverse appears to be the case ; for the composition of the skeleton 

 is fairly constant in its simplicity, but the cirrhi vary considerably both in number 

 and in size. 



In a fully developed cirrhus of Act. polymorpha (PL III. fig. 8 a) the number of 

 segments varies from 11-11, being usually 12 or 13, the last of which is in the form of 

 a stroug sharp claw. This is attached by simple suture to the penultimate segment, 

 which is prolonged at the base of the claw into a short opposing process on its concave 

 or aboral margin. 



The diameter of the basal segment somewhat exceeds its length ; but in the second and 

 third segments this disproportion between the length and breadth is reduced, and in the 

 fourth it becomes reversed, the length of this segment being slightly greater than its 



1 rhil. Trans, he. cit. p. 704. 2 Beitrage <fcc. he. cit. p. 40. 



