86 INTRODUCTION. 



/ 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 (Tere- 

 bratula). 



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 a toothed tongufc. 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 

 respiration. 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. 

 SUB-KINGDOM VI. VERTEBRA TA. 



Body composed of a number of definite segments arranged longitudi- 

 nally, 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 back- 

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



CLASS I. PISCES (Fishes). Breathing- organs in the form of gills. 



