Introduction to Animal Morphology. 135 



sile organs {pedicellaricL). Each consists of a short pillar 

 supporting two or three movable blades or valves, which are 

 usually open, but can snap together like a crab's pincers ; 

 some have two narrow-pointed (p. forcipata), or two or three 

 broad, overlapping blades (p. valvata), and they may be 

 sessile, as in Llwydia. The mouth is central, contractile ; the 

 stomach is a sac with numerous (usually ten) lateral coeca, 

 each of which is tied to the dorsal surface of the body cavity 

 by a radial membranous fold, and consists of a stem giving 

 off 30-50 short, side branches, each ending in 6-8 vesicles 

 containing a yellow or brown bitter fluid (bile ?) ; two of 

 these stretch into each arm. The oral part of the stomach 

 may become everted as a sucking organ, paralysing its prey 

 by some secretion. Deslongchamps saw five everted soft 

 vesicles inverted into the mouth of a Mactra, which was being 

 devoured. McAnd^-ew saw a similar sac attached to the 

 hinder end of a Littorina. There is no anus in Astropecten, 

 Ctenodiscus, or Llwydia ; in the others it is central, sub- 

 central, or to the left of the madreporiform plate in an inter- 

 brachial pore. In Astropecten there are two non-ciliated 

 inter-radial coeca opening dorsally into the stomach ; these 

 are absent in Llwydia, five in Archaster, five bifid ones in 

 Culcita ; they are supposed to be hepatic or renal, but 

 contain neither uric acid nor ammonia. The nerve ring is 

 broad and flat, and may possess ganglion cells. Each radial 

 branch is covered by a layer of ganglion cells, and dilates to 

 the middle of the ambulacral space, being sometimes gutter- 

 shaped, the demi-canal being completed into a tube by fine 

 cellular tissue on the dorsal side. At the end of each ambu- 

 lacral row the nerve often leaves its groove, and gives off" one 

 twig, ending in a gangliform swelling beneath the eye, and 

 another to an extensile, ciliated feeler {Greef). The eyes are 

 near the ends of the rays, and consist of 80-200 small, conical, 

 stalked pigment granules, with crystal rods (?) surrounded by 

 gelatinous tissue, lying under a common cornea (?). 



The ambulacral system has a thick-walled oral ring, 

 smooth, ciliated within, and surrounded by circular fibres 

 externally ; the pedicelli are conical in the aproctous, cylin- 

 drical in the proctuchous forms. Each of the five Polian 



