566 THE ANIMAL KINGDOM. 



tube-foot is azygos and the first of the series developed. It forms the 

 so-called tentacle, and has no disc. 



The ciliated digestive tract consists of an oesophageal (cardiac) division 

 with short radial pouches, which are eversible and are retracted by special 

 muscles ; of a stomach (pyloric division), from which caeca radiate into the 

 arms, and a tubular intestine which opens by an anus placed dorsally and 

 a little excentrically. The anus is absent, however, in all the Astropec- 

 tinidae (Astropecten, Ctenodiscus, Luidia, Leptychaster] the genus Archaster 

 excepted. The stomachal caeca commence as five narrow tubes, each of 

 which divides once, and the ten branches thus formed bear numerous 

 lateral branches with ampullae. They are suspended to the dorsal perisoma 

 of the arms by double mesenteries. The intestine has close to the anus 

 two interradial ampullae sometimes five, each of which may divide once 

 (Culcitd). The anus in Zoroaster is placed interradially within the circle 

 of under-basals. The digestive tract is attached by a dorsal mesentery to 

 the body walls. The coelome is ciliated and extends into the arms. These 

 extensions are narrow in Brisinga, in all others large. The generative 

 glands are generally ten in number, and are placed interradially. They 

 consist of branched caeca with a common duct, which, as a rule, opens 

 dorsally by one or, more rarely, by many pores on a calcareous plate placed 

 at the base of the arm interradially, and called in the latter case a 

 sieve-plate. When mature the glands extend into the cavities of the arms. 

 There is sometimes a greater or less number of genital glands extending 

 up each side of the arms and opening by separate ducts, a condition most 

 perfectly developed in Brisinga. A blood space surrounds the glands, 

 and it is possible that an infertile rhachis may extend to the aboral blood- 

 ring, as in Ophiuroidea. 



The free swimming larva is known as Bipinnaria or Brachiolaria. 



Most Asteroidea live in shallow water, but there are a number of deep- 

 water genera which possess primitive or peculiar characters (Zoroaster, 

 Brisinga, &c.). The group appears in the lower Silurian. 



The Palaeozoic Asteroidea are classified as Encrinasteriae, characterised by the 

 alternate arrangement of the ambulacral ossicles. In all living Asteroidea they are 

 opposite. Viguier proposes (A. Z. Expt. vii. 1878) to group the latter as 



1. Asteriae ambulacrariae, with the mouth plates ambulacral, the pedicellariae 

 pedunculate, straight or crossed, and the ambulacral pores as a rule arranged in 

 zigzag, so that there are apparently two rows of feet. Asteriadae, Heliasteridae, 

 Brisingidae. 



2. Asteriae adambulacrariae, with mouth-plates adambulacral, sessile pedi- 

 cellariae, and the ambulacral pores as a rule in a straight line. Other Asteroidea. 



See lit. pp. 194, 196. 



Brisingidae of the ' Tatisman,' Perrier, C. R. 101, 1885. 



Haematoporphyrin in integument, MacMunn, Journal of Physiology, vii. (3), 

 1886. 



