ECHINODERMATA. ASTEROIDEA. 69 



given off; these two circular vessels are connected by 

 means of a tube passing along the stone-canal, and some- 

 times regarded as a heart*. The genital glands occur in 

 pairs at the base of each arm and they open to the 

 exterior between the rays on the dorsal surface. 



ORDER. ENCRINASTERI^. 



The ambulacral ossicles alternate on either side of the 

 ambulacral groove. 



Palaeaster. Arms five, thick, convex, short or of 

 moderate length. The dorsal surface formed of rows of 

 small ossicles provided with spines. There is a row of 

 adambulacral plates and also a row of large marginal 

 plates. Madreporic tubercle small. Bala Beds to Car- 

 boniferous. 



ORDER. EUASTEROIDEA. 



The ambulacral plates are placed opposite each other. 



Astropecten. Body flattened. Arms five, long. 

 Two rows of marginal plates, the ventral row carries 

 spine-like scales, which increase in size outwards and 

 terminate in long spines ; the dorsal row granulated and 

 often spinous. Dorsal surface of the disc and rays fur- 

 nished with small appendages which are covered with 

 bristly prolongations. Anus absent. Lias to present day. 



Distribution of the Asteroidea. 



The Asteroidea have a wide distribution in the ocean 

 at the present day ; they are most abundant at moderate 

 depths, but also occur in abyssal regions. 



The fossil forms do not differ greatly from the modern 



* A different interpretation of the vascular system has been given 

 by CuSnot. 



