MOLLUSCA. 



323 



The velum at the same time becomes drawn out into lobes, and 

 we get the well-known veliger larva. A point to notice about the 

 development is that the blastopore often assumes the form of a slit 

 which may completely close, or close behind and remain open in 

 front as the mouth, or less often close in front and remain open 

 behind as the anus. 



The Mollusca are essentially aquatic animals, and especially 

 marine ; only a few live on land, and these seek damp situations. 

 They are one of the largest and most diversified of animal groups, 

 and apparently have always been so since the earliest fossiliferous 

 periods, for all of the great groups are represented in the Palaeozoic 

 period, and a Helix, or a form allied to Helix and belonging to the 

 most specialized of Mollusca is now known even from the Car- 

 1 tonif erous. 



At the present day about 25,000 species are known; they are 

 distributed over the whole surface of the earth, and are found in 

 the sea to a depth of nearly 3000 fathoms. Their habits of life 

 are most various; there are parasitic forms, Entoconcha, Eulima, 

 Entocola.c, Entovalva, Stilifer, Tltijca (all on Echinoderms) ; com- 

 mensals (e.g., Montacuta) ; fixed forms ( Vermetus, Ostrea) ; pelagic 

 forms; and creeping forms, the latter constituting the majority. 



Their duration of life, where known, varies from one to thirty 

 years ; the Pulmonates generally live two years, but the garden-snail 

 has been known to live five years. The oyster is adult at about five 

 years, and lives to ten years. The Anodonta do not arrive at sexual 

 maturity till five years, and live for twenty or thirty years. 



The following is the classification of the Mollusca adopted in 

 this work : 



CLASS I. LAMELLIBRANCHIATA. 



II. SCAPHOPODA. 



,, III. SOLENOGASTRES. 



IV. GASTROPODA. 



Sub-class 1. ISOPLEURA. 

 ,, 2. AN ISOPLEURA. 

 Order 1. Streptoneura. 

 Sub-order 1. Aspidobranchiata. 



Tribe 1. DOCOOLOSSA. 

 ,, 2. RHIPIDOGLOSSA. 

 Section A. Zygobranchiata. 

 ,, B. Azygobranchiata. 



Sub-order 2. Pectinobranchiata. 



Tribe 1. PTENOGLOSSA. 

 2. RACHIGLOSSA. 



CLASS IV. GASTROPODA Continual. 

 Tribe 3. TOXOOLOSSA. 

 4. TAENIOGLOSSA. 

 Section A. Platypoda. 

 ,, I!. Heteropoda. 



Order 2. Eutliyneura. 



Sub-order 1. Opisthobranchiata. 



Tribe 1. TECTIBRANCHIATA 



(includes Pteropoda). 



2. NUDIBRANCHIATA. 



Sub-order 2. Pulmonata. 

 Tribe 1. BASOMMATOPHOEA. 

 ,, 2. STYLOMMATOPHORA. 



CLASS V. CEPHALOPODA. 

 Order 1. Dibranchiata. 



2. Tetrabranchiata. 



