CLASSIFICATION. 17 



by a complicated system of calcareous plates, giving a 

 rigidity to the general form of the animal. 



The plates constituting the dorsal system may be very 

 numerous and complicated, so as to form a stem by which 

 the animal is invertedly attached to the ground, while the 

 ambulacral system may be limited to a comparatively small 

 area, and the interambulacral system is wanting, as in Coma- 

 tula (sea lily); the dorsal system, though large, may not form 

 a stem, yet fold over on the under side of the animal so as to 

 enclose entirely the ambulacral system, forming a kind of 

 shield to the bases of the long snake-like arms, as in Ophio- 

 pholis ; the system may occupy the back of the animal, but 

 not to enclose the ambulacral suckers which find their way 

 between the rows of small plates along the under side of the 

 arms, as in Asteracanthion (star fish) ; the system may be 

 contracted to a small area on the top of the animal, the rows 

 of interambulacral plates which are separated and lie on 

 either side of the ambulacra in the star fish may now be 

 united, and both ambulacral and interambulacral systems 

 bent upward, meeting in a small dorsal area above, as in Tox- 

 opneustes (sea urchin) ; or the ambulacral and interambulacral 

 systems may take a great preponderance over the dorsal sys- 

 tem, the latter being pushed out to the end of a cylinder, 

 while the two former run along its whole length, as in Holo- 

 ihuria (sea cucumber). (E. C. & A. Agassiz.) 



SAC (Mollusca). 



A mollusc has the stomach and viscera enclosed by a fleshy 

 sac. Principal nerve masses, consisting of ganglia, which 

 are adjacent to, or surround the oesophagus. Intestine bend- 

 ing inward, or having an outward flexure. Heart on outer 

 side of intestine. (Morse.) 



Divided into Polyzoa, Brachiopoda, Tunicata, Lamellibran- 

 chiata, Gasteropoda, Pteropoda, and Cephalopoda. 



Polyzoa. Compound molluscs. Respiratory apparatus 

 exsertile from sac in the form of ciliated tentacles around 

 mouth. Intestinal canal definite, curved upon itself. Single 

 ganglion situated at re-entering angle. Heart, none. 



Brachiopoda. Animals solitary. Mantle covered by bi- 



