CONTENTS 



CHAPTER VII 



BACKBONELESS ANIMALS (INVERTEBRATA). STRUCTURE AND 

 CLASSIFICATION OF NEMERTINES AND MOLLUSCS 



Page 



Distinctive Characters of Higher Invertebrates as compared vi^ith Vertebrates 302 



Main Groups or Phyla of the Invertebrates - 3°4 



NEMERTINES (Nemertea) 

 Structure of a Typical Nemertine - - 3°S 



MOLLUSCS (Mollusca) 



Structure of the Ormer or Sea-Ear (Haliotis tuberculata), taken as a Type 2P7 



Essential Characters of Molluscs 311 



Class I.— HEAD-FOOTED MOLLUSCS (Cephalopoda) 311 



Structure of the Common Cuttle-Fish (Sepia officinalis), taken as a Type - 311 



Sub-Class I. CUTTLE-FlSHES AND OcTOPI (Dibranchiata) 314 



Sub-Class II. Pearly Nautilus (Tetrabranchiata) - 316 



Class 2.— SNAILS AND SLUGS (Gastropoda) 317 



Sub-Class I. — Streptoneura or Fore-gilled Snails (Prosobranchia) 318 



Order I. — Comb-GILLED Snails (Ctenobranchia) 318 



Structure of the Periwinkle (Littorina littorea) and Purple-Shell (Purpura 



lapillus), taken as Types. 

 Whelks, Mitre-Shells, Volutes, Olive-Shells, Harp-Shells, Cone-Shells, 

 Turret-Shells, Wing-Shells, Helmet-Shells, and Covi'ries; Heteropods. 

 Order 2.— Shield-gilled Snails (Aspidobranchia) 322 



Trochida^, Ormers, Limpets. 

 Sub-Class 2. — EuTHYNEURA 324 



Order i. — Hind-gilled Snails (Opisthobranchia) 324 



(i) Tectibranchs (Bubble-Shells, Sea-Hares, Wing-footed Snails or 

 Pteropods) — (2) Nudibranchs (Sea-Slugs). 

 Order 2. — LuNG Snails (Pulmonata) 326 



Structure of the Garden Snail (Helix aspersa), taken as a Type - 326 



Land Slugs, Fresh-vi'ater Pulmonates. 



Class 3.— BIVALVE MOLLUSCS (Lamellibranchia or Pelecypoda) 328 



Structure of Fresh-water Mussels (Unio and Anodonta), taken as Types 328 

 (i) Cockles — (2) Gapers — (3) Razor-shells — (4) Rock-borers — (5) Sea Mussels — 



(5) Ark-Shells — (7) Scallops — (8) Oysters — (9) Nucula Family 333 



Class 4.— TUSK-SHELLS (Scaphopoda) 338 



Structure of the Common Tusk-Shell (Dentalium vulgare). 



Class 5.— PROTO-MOLLUSCS (Amphineura) 339 



Mail-Shells (Chitons), Worm-like forms (Neomenia, Proneomenia, Chasto- 

 derma). 



CHAPTER VIII 



STRUCTURE AND CLASSIFICATION OF JOINTED-LIMBED 

 ANIMALS (Arthropoda) 



Essential Characters. Subdivisions , 342 



