EOACTINID^E. 185 



1905, ' Asteroidea of the Albatross,' ' Mem. Mus. Comp. Zool. Harvard,' vol. xxxiii, 

 fig. 51, Pentagonaster emesti, fig. 135, Paulia horrida; Koeliler, 1910, 'Shallow- 

 water Asteroidea in the Indian Museum,' pi. xi, fig. 3, Pentaceros indicus, pi. xii, 

 fig. 2, P. reinhardfi, and others; Gregory, op. cit., fig. 1/;, Lindstrqmaster, the ray 

 in the north-west position." 



I am inclined to think that originally the adambulacralia of the Asterozoa were 

 even more like covering-plates than in the examples quoted by Bather. We have 

 seen that in several genera, Urasterella and its allies, Lepidaster and Lepidactis, the 

 adambulacralia are large and could completely cover the groove. The specimen 

 of Palasterina primseva figured in PI. XV, fig. 7, shows a similar condition. In 

 the Middle Ordovician species Schuchertia stellata and the Upper Ordovician 

 8. laxata (figured below) the adambulacralia completely cover the groove, while 

 in the Middle Silurian form 8. werilocki the adambulacralia are comparatively 

 small and the groove is widely open. A similar reduction in the size of the 

 adambulacralia can be followed in the Asteroidea of Section A (compare, e. g., 

 Belaster ordoricus, PI. Ill, fig. 3, and Proi nopal xaster elizas, PI. IV, fig. 2). This 

 evidence goes to show that there is in some groups a progressive loss in size of the 

 adambulacralia which disguises their original cover-plate form. This evidence 

 might lead one to agree with Bather that Edrioastev and the Asterozoa descended' 

 from a common ancestor. 



There is, however, a feature of the adambulacralia of the Asterozoa, namely, 

 the adoral tip, which requires further investigation, as it may have a distinctive 

 value. It has been noticed for Hudsonaster and Urasterella by Hudson (93, 98). 

 In my account of the Urasterellidae (see p. 133) I assumed that the tip arose 

 because of the extra muscular power given to the adambulacral muscles. I find 

 that the tip is exceedingly common in the Asterozoa. It may be seen in the 

 recent Archaster (Text-fig. 125), in Schuchertia (Text-fig. 124), and in many other 

 species. I have looked through Bather's descriptions to see if there is a similar 

 appearance in the Edrioasteroidea, and find that in the description of Edrioaster 

 bigsbyi (89, p. 1(52) he states that " the cover-plates abut closely, in tesselate 

 fashion, when closed in the normal position over the radial groove. When pressed 

 inwards, however, as in B, they have sometimes been made to imbricate with 

 adoral overlap. This suggests a possibility of the converse action, namely, that 

 in life they may have assumed a similar imbricate arrangement when they opened 

 outwards, thus forming a slight gap between adjacent plates." The point is an 

 interesting one, and would repay further investigation. 



So far we have dealt with the ambulacralia and adambulacralia as flooring- and 

 covering-plates of the groove. It now remains to consider them as "bones" to 

 which are attached the muscles concerned in arm-movement. 



The arm-movements are of two kinds — (1) flexion downwards or upwards in a 

 vertical plane ; (2) lateral flexion. In the recent Asteroidea these movements are 



25 



