12 6 Lifroduction to A 7iimal Morphology. 



mouth. Close to this digestive tube a discoidal mass 

 of protoplasm forms, first on one side, then extending 

 around it ; then, according to Midler^ an involution 

 of the integument at one side of the back forms a 

 pouch, which narrows into a tube, whose fundus 

 dilates, in the protoplasm around the stomach, into a 

 rosette, extending around the oesophagus as a circular 

 vessel, from which the rudiments of the ambulacral 

 canals and pedicels arise. The primary involution 

 narrows into a tube, the stone- canal or hydrophore, 

 which leads from the outer ab-oral side into the ring- 

 canal ; its outer opening becomes closed by the for- 

 mation of a perforated, often labyrinthically tubulated, 

 calcareous scale, the madreporiform plate. The later 

 researches of J/f/i-f^/zw/y^Y^^ and Agassiz indicate that 

 the perivisceral cavity, as well as the ambulacral 

 canals, are formed as offshoots from the digestive 

 canal. A symmetrical larval skeleton of spicules 

 may exist (except in Asteriadae and Holothuriadae), 

 no part of which is transferred to the adult. When a 

 coeloma exists in the larva, it is not continued into the 

 perivisceral cavity of the adult.* 



This sub-kingdom is divisible into three classes : — 

 Class i . Stellerida — Starfishes, having a central 

 disc, and usually 5-20 diverging arms. The peri- 

 stome is not calcified through its entire thickness, and 

 consists of: — ist, a leathery papillary layer, whose 

 spinose process may be clustered, bristle-like points 

 (paxillce), or echinulated, one-jointed spines. 2nd, a 



* Prof. Huxley originally united these and the Scolecida under the 

 name Annuloida, a convenient, though unnatural, grouping, as the 

 Echinoderms are Deuterostomes, while most of the Scolecides are Archjeo- 

 stomatous. Such a group would consist of animals with a separate ali- 

 mentaiy canal, a water-vascular system, and a thread-like nervous system. 



