60 BULLETIN OF THE 



NOTE ON THE STRUCTURE OF ASTROPHYTIDiE. 



In very early youth the Astrophytons bear a close resemblance to true 

 Ophiurans, but they rapidly change with growth. Their structure will be 

 more fully treated in the main work, and only two or three points of differ- 

 ence will here be suggested. 



First, as to the arm covering. The young tip of an Astrophyton twig has 

 the side arm-plates quite encircling it (Fig. 495), just as in an Ophiuran ; but 

 already at the base of the same twig this plate is quite subordinate (Fig. 494 i), 

 while at the base of the arm (Fig. 493 i) it occupies only the under surface, 

 while the arm has risen in a high arch above it. It is not otherwise in the 

 simple-armed Astroschema (Fig. 491 i). The upper arm-plates have no regular 

 form, or stated mode of division ; but doubtless they are represented by a casing 

 of very irregular scale-like pieces, to be found on the terminal branches of 

 Astrophyton, and in the narrow belts of broken plates found in Astroschema 

 (Fig. 491). The under arm-plates are extremely variable ; in the type of 

 Euryale asperum they are essentially in one piece, and are constant to the end 

 of the branches (Fig. 499 h), while for the type of Astrophyton costosum they 

 are quite wanting, except perhaps the first one (Fig, 497 A), and are replaced 

 by the large side arm-plates (Fig. 497 i) ; in the cold-water Astrophytons, such 

 as A. Agassizii, they are plainly distinguished in the young, though divided in 

 three pieces (Fig. 492 h, h). To such a structure of arm-plates the nearest 

 approach among Ophiurans would perhaps be Ophiomyxa. 



Secondly, as to the arm-spines. There are found, at the extreme tip of a 

 twi« of Astrophyton (Fig. 495), little booklets on the side arm-plates ; when 

 the arm has risen above the plate, and become quite distinct from it, there 

 are found two or more large hooks (Fig. 494 q), which are the homo- 

 loo-ues of tentacle-scales, and which, nearer the base of the arm, usually become 

 blunt spines (Fig. 493 q). In addition to these there are found on the twigs, 

 in the true Astrophytons, Astroclon^ Astrocnida, and among the simple-armed, 

 in Astrogomphus, Astroporpa, Astrochele, and Astrotoma, two zones or belts of 

 raised grains, each grain bearing a booklet (Fig, 494). These belts of hook- 

 bearing grains are therefore characteristic of a group among Astrophytidce ; 

 while another is destitute of them, as Euryale asperum (Figs. 500, 501), Tri- 

 chaster, Astroceras, Astroschema (Fig. 491), Ophiocreas, and Astronyx. 



Thirdly, the mouth-shields among Astrophytons are quite subordinate, al- 

 though so important among Ophiurans. Frequently there is but one (Fig. 

 492 a), and the position is very variable. The side mouth-shields, on the con- 

 trary, are usually very prominent (Figs. 492, 497, 499 h) ; so large are they in 

 Trlchaster that Midler and Troschel mistook them for a mouth-shield cut in 

 two. The entirely different structure of Euryale asperum as exhibited in the 

 figure (499), and especially the elongated side arm-plates (Fig. 501 i), absence 

 of hook-bearing grains, and distinct build of mouth and under arm-plates> 

 makes it advisable to remove the species from Astrophytoji and restore to it the 

 name Euryale. It is a question, also, whether the tropical Astrophytons 



