196 BRITISH PALAEOZOIC ASTEROZOA. 



enlarged stomach. The Asteroidea have a relatively small stomach. The}- require 

 little bulk of food, but large digestive glands. It is possible that as these glands 

 are situate at the bases of the arms, the enlargement of the glands, due to a 

 perfection of the digestive apparatus, may have been a primary cause of the 

 approximation and final fusion of these arm bases. We know, e. g., that the 

 power of the digestive juices of Asterias is such that the lips of the everted 

 stomach can digest a lamellibranch on touch, and that it is not necessary for the 

 prey to be taken within the mouth-cavity (see p. 11). A further contributory 

 cause of the enlargement of the arms at the base is suggested by the following 

 remarks of Verrill (103, p. 32), who states in respect to the recent Asteriidae : 

 " Moreover, in most species, the rapid increase of the dorsal skeleton alone 

 apparently does not give sufficient space for the rapidly growing internal organs, 

 especially the reproductive organs, within the bases of the rays, and therefore new 

 rows of plates (figs, i, iii, iv) must be interpolated between the infero-marginals 

 and the adambulacrals, and sometimes between the upper and lower marginals, to 

 increase the diameter of the rays." In other words, mere increase in size of the 

 reproductive organs causes a widening, approximation, and possible final fusion of 

 the arm bases in the Asteroidea. Space for the reproductive organs is found, in the 

 Ophiuroidea, within the already enlarged interradial disc areas, as may be seen by 

 reference to Text-fig. 134, p. 193 (compare Text-fig. 214, p. 490, MacBride, 43). 



It is clear from these considerations that a study of the mouth-frame of the 

 fossil Asterozoa may give one important clues as to habits, and I am giving there- 

 fore a series of oral views (Text-figs. 136 — 138) as standards for comparison. 



The apical views figured on the previous pages are often difficult to obtain, 

 but the corresponding oral region usually can be exposed readily, even when it 

 lies at the bottom of a deep, narrow groove. Nearly all the fossil material is 

 preserved as moulds, and the gutta-percha casts can be bent whilst still warm and 

 the groove opened out. Maceration in potash will give corresponding views for 

 recent forms. 



Text-figs. 136 and 137 were drawn from species of Astropecten after macera- 

 tion in potash. These figures show the junction of the ascending limb and the 

 first ambulacral very clearly. The important points to notice in the figures are 

 the position of the groove for the water-vascular ring (just at the junction of the 

 ascending limb and the first ambulacral) and the nerve-ring (just below the oral 

 surface of the mouth-angle plate). The area between the two grooves is that 

 occupied by the wing-shaped apophysis. A similar oriented view (Text-fig. 138) 

 of the Silurian form Palasterina primasva is given. The correspondence between 

 the recent and fossil forms in all essential details is very exact. The only 

 important difference is that no ambulacral pores are visible in Palasterina and the 

 ascending limb of the mouth-angle plate has not as yet become firmly ossified with 

 the first ambulacral. 



