42 INVERTEBRATE ZOOLOGY. 



Order 4. Opisthobranchiata Shell delicate or wanting. Respiration 



seldom by ctenidia, often bj^ secondary 

 or adaptive gills, or through the integu- 

 ment. Gills, when present, lie behind 

 the heart. Back of naked forms often 

 ornamented with simple or dendritic 

 papillae. Hermax^hroditic. 

 i^ Bulla, Eolis. 



Order 5. Pteropoda Shell fragile or wanting, foot developed 



into a pair of wing-like expansions. 

 Hermaphroditic. A small groiip of forms 

 which swnm at night npon the surface of 

 the ocean— often referred to preceding 

 order, from which they have iindoiibt- 

 edly been derived. -^ Cymhuliopsis. 



Class V. Cephalopoda Body bilaterally symmetrical, extended 



in a dorso-ventral direction and flattened 

 antoro-posteriorilj' so that the anterior 

 aspect seems dorsal, the posterior ven- 

 tral, the dorsal posterior, etc. The 

 greatly modified foot forms a series of 

 tentacles about the mouth and a funnel 

 or infundibiilum behind it. The head is 

 large and distinct, with two large prom- 

 inent eyes. Mouth provided with a pair 

 of chitinous jaws. Shell large and cham- 

 bered, or reduced or even internal. 



Order 1. Tetrabranchiata Gills four, mouth surroimded by niimer- 



ous unarmed tentacles. Ink bag fails. 

 A heavy external shell convoluted and 

 divided into chambers — the animal being 

 in the terminal and largest one. 

 -^ NautihiH (only livhig form). 



Order 3. Dibranchiata Gills two. Eight (or ten) arms around 



the mouth, covered with cup-shaped 

 sucking discs. Ink bag present. Shell 

 internal (a very fragile external shell in 

 ^rgonuuta). i^ Octopus, Loligo, Sepia. 



