28 BRITISH PALAEOZOIC ASTEROZOA. 



strainer to prevent the entrance of coarse particles. To permit tliis extensive 

 movement the articulatory facets on the proximal surface of the first vertebra have 

 been much modified; the median knob and pit have disappeared, and the dorso- 

 lateral pits are raised on to the surface of processes, so that there are in all four 

 processes, two of which articulate with one half of a jaw. The mouth can be 

 narrowed and the jaws forced inwards towards the centre by the simultaneous 

 contraction of five muscles each, which unite the two halves of a jaw," 



The buccal tentacles, of which mention has been made, are situated in cups on 

 each jaAV (Text-figs. 25, 27, p. 32). The first buccal tentacle points upwards and 

 the second forwards. The position of each has, therefore, become modified from 

 that observed in Asteroids, in which the proximal tube-feet play little part in feed- 

 ing. Both buccal tentacles are supplied by a single branch, which comes directly 

 off the water-vascular ring and bifurcates in the substance of the ossicle (Text- 

 fig. 25). A distinct excavation for the nerve-ring is also noticeable. 



The Homologies between the Mouth-parts of Recent 

 Asteroidea and Ophiuroidea. 



(i) Those suggested b)/ EmWijologt/. — The homologies until recently accepted by 

 the majority of writers were those suggested by the classic researches of Ludwig, 

 who arrived at his conclusions by investigations of the young stages of Recent 

 forms. According to Ludwig the mouth-parts of both Asteroids and Ophiuroids 

 are derived solely from modifications of the ambulacralia and adambulacralia. 



It is perhaps easiest to visualise Ludwig's interpretation by reference to Text- 

 fig. 3, p. 12. It has already been noticed that the first ambulacralia of the Asteroidea 

 are much larger than the remaining ambulacralia, and that each of these first 

 ambulacrals possesses two processes. If we draw a line across the base of the 

 anterior process we may regard the first ambulacra! as having two components, 

 one of which corresponds to the mouth-angle plates and. the second, to the first 

 recognisable adambulacral. The anterior process would then be ay and the mouth- 

 angle plate ad^, the posterior process would be a.^ and the first recognisable 

 adambulacral ado. 



Although Ludwig uses a diagram of this kind (compare 40, fig. 5, p. 530), he 

 makes no claim that the anterior process is a true ambulacral formed by the 

 natural growth of the embryonic ambulacral which occupies its position in the 

 young stages. As he himself shows, the anterior process is formed secondarily by 

 a downward growth from the body of the large ambulacral (Ludwig's (^o), which 

 meets an upwardly projecting growth from the mouth-angle plates (Ludwig's ndi). 

 For all practical purposes we may therefore state that Ludwig's embryonic a^ is 

 not represented in the mouth-frame of the Recent Asteroidea. 



