CHIEF DIVISIONS OF THE ANIMAL KINGDOM. 49 



system consisting of a single ganglion or of scattered pairs of ganglia. A 

 distinct heart and breathing-organ, or neither. 



The Mollusca may be divided into the two following primary divisions, 

 containing the following classes : 



A. MOLLUSCOIDA. Nervous system consisting of a single ganglion or of 

 a principal pair of ganglia. No heart, or an imperfect one. 



CLASS I. POLYZOA. Animal always forming compound growths or 

 colonies. No heart. The mouth of each zooid surrounded 

 by a circle or crescent of ciliated tentacles. Ex. Sea- 

 mats (Flustra), Lace-coral (Fenestella). 



CLASS II. TUNICATA. Animal simple or compound, enclosed in a 

 leathery or gristly case. An imperfect heart. Ex. Sea- 

 squirts (Ascidia). 



CLASS III. BRACHIOPODA. Animal always simple ; the body enclosed 

 in a bivalve shell. Mouth furnished with two long fringed 

 processes or "arms." Ex. Lamp-shells (Terebratula). 



B. MOLLUSCA PROPER. Nervous system consisting of three principal 

 pairs of ganglia. Heart well developed, consisting of at least two chambers. 



CLASS IV. LAMELLIBRANCHIATA (Bivalve Shell -fish). No distinct 

 head ; no teeth. Body enclosed in a shell which is " bi- 

 valve," or composed of two distinct pieces. One or two 

 leaf-like gills on each side of the body. Ex. Oyster 

 (Ostrea), Scallop (Pecten), Mussel (Mytilus). 



CLASS V. GASTEROPODA. A distinct head and toothed tongue. Shell 

 absent in some, but mostly present, and usually consisting 

 of a single piece ("univalve"). Locomotion effected by 

 creeping about on the flattened under surface of the body 

 (" foot "), or by swimming by means of a fin-like modifi- 

 cation of the same. Ex. Whelks (Buccinum), Limpets 

 (Patella), Sea-lemons (Doris), Land-snails (Helix), Slugs 

 (Limax). 



CLASS VI. PTEROPODA. Animal oceanic, swimming by means of two 

 wing-like appendages, one on each side of the head. Size 

 minute. Ex. Cleodora. 



CLASS VII. CEPHALOPODA. Animal with eight or more arms, placed 

 in a circle round the mouth. Mouth armed with jaws, and 

 and a toothed tongue. Two or four plume-like gills. In 

 front of the body, a muscular tube ("funnel") through 

 which is expelled the water which has been used in respira- 

 tion. An external shell in some, an internal skeleton in 

 others. Ex. Calamaries (Loligo), Cuttle-fishes or Poulpes 

 (Octopus), Paper-Nautilus (Argonauta), Pearly Nautilus 

 (Nautilus). 



VERTEBRATE ANIMALS. 



6" UB - KINGDOM VI. VER TEBRA TA . 



Body composed of a number of definite segments arranged longitudinally, 

 or one behind the other. The main masses of the nervous system are 

 placed on the dorsal aspect of the body, and are completely shut off from 

 the general body-cavity. The limbs (when present) are turned away from 

 that side of the body on which the main nervous masses are situated, and 

 are never more than four in number. In most cases a backbone, or 

 "vertebral column," is present in the fully-grown animal. 

 D 



