REPORT ON THE CRIXOIDEA. 83 



In man}' of the tropical Comatulse the pinnule-ambulacra are fully as well or even 

 better plated than those of the Pcntacrinidai. Antedon acoela and Antedon iiuecfialiii,^ 

 both from Stations where Pentacrinida) are abundant, are good instances of this (PL LIY. 

 figs. 4, 6-9). The differentiation of side plates and covering plates is more complete ; and 

 the plates themselves are not only larger relatively to the pinnule-joints, but also absolutely 

 so. When the covering plates are erected and the groove opened, as shown in Antedon 

 incequcdis (PI. LIV. fig. 8), the tentacles are extended between them. But the tentacles 

 can be completely retracted and the plates closed down so as to convert the grooves into 

 tunnels, as shown in Antedon acoela (PL LIV. fig. 4). 



In the distal edge of each of the side plates is a small rounded notch, so that there 

 is a series of gaps along each side of the ambulacrum, one between Qwavy two plates. 

 These lodge the problematical " sacculi " which are so characteristic of the g&mx?, Antedon. 



In those Comatulas with plated pinnules which have short and rounded genital 

 glands, instead of the long fusiform structures characteristic of Antedon eschrichti, Hyo- 

 crinus, and the Bourgueticrinidaj, there is sometimes a curious modification of the 

 perisomatic skeleton on the genital pinnules. The enlarged part of the pinnule is 

 protected by a very strongly developed anambulacral plating, which is much more 

 regular and closely set than that of the disk and arm-bases, especially iu the case of 

 Antedon acoela (PL LIV. figs. 1-3). Resting upon the four or five middle joints of the 

 short pinnules there is a double row of large plates, which are rectangular at the base 

 but somewhat more irregular in shape at their upper ends. There are generaUy five or 

 six plates in each row ; but those of the two sides have no fixed relative positions, 

 sometimes corresponding exactly, and sometimes alternating as exactly. They have 

 the same protective function and very much the same appearance as the large side plates 

 of Hyocnmcs (PL Vc. figs. 9, 10, sij), but difier from them in two ways. In other 

 species of Antedon, such as Antedon angusticalyx and Antedon incerta (PL 1A\. 

 figs. 5, 6), these protecting plates are smaller and more irregular than in Antedon acoela; 

 while in Antedon incerta the two rows are separated by the ambulacrum with its well 

 developed side and covering plates. But in Antedon angusticalyx and iu Antedon acoela 

 these swollen lower pinnules receive no branches from the brachial food-groove, just as in 

 many species of Actinometra ; and the anambulacral plates covering the genital glands 

 consequently meet one another in the medio-ventral line of the pinnule above the gland 

 within. The sacculi, however, which lie at the sides of the ambulacra may extend on 

 to these grooveless pinnules, and occupy small holes between the large protecting plates ; 

 while in the outer joints of the pinnules, beyond the glands, the sacculi occupy the 

 median gTOOvc on the upper surface of the skeleton, as is well shown iu Antedon angusti- 

 calyx (PL LIV. fig. 5). 



a'' 

 1 The specific formula of this type is— A . 3(1 . or 2)) . ) ^ . 



