428 



CHAPTER XVI. 

 PROSOBRANCHIATE GASTEROPODS. 



IN the Prosobranchiate division of the Gasteropods the branchiae or 

 gills are composed of numerous leaflets cut like the teeth of a comb, 

 and attached, by one or many lines, to the upper part of the respiratory 

 cavity. They constitute the most numerous order of Cephalous 

 Molluscs, comprehending nearly all the univalve spiral shells, and 

 many others which are simply conical. They inhabit the sea, rivers, 

 and lakes, and are of all sizes. The most remarkable genera which 

 we shall describe belong to the family Trochdida and Buccinoida. 



This, the fourth order of Gasteropods, Prosobranchiata, includes 

 the Pectinibranchiata and three other orders of Cuvier ; in it the sexes 

 are distinct, the branchiae pectinated or plume-like, situated (proson) in 

 advance of the heart, and the mantle forms a vaulted chamber over 

 the back of the head. It is divided into two sections and twenty-one ' 

 families. The first section, Holostomata, contains the sea-snails, where 

 the margin of the aperture of the shell is entire. The muzzle is short, 

 non-retractile, and they are mostly phytophagous. The second section 

 contains the carnivorous Gasteropods. The aperture of the shell is 

 notched or produced in front ; animals with a retractile proboscis. 

 Unlike the first section, the species are all marine. 



The first family is that of the Chitonida. The Chitons are very 

 singular creatures, destitute of eyes and of tentacles ; they bear upon 

 their back in place of a shell a cuirass composed of imbricated and 

 movable scales. They have the power of elongating and contracting 

 themselves like the snails. They roll themselves up into a ball like 

 the woodlouse. They adhere with great force to the rocks, preferring 

 those places most exposed to the beating waves. Chiton magnificus 

 (Fig. 210) is widely distributed. 



The second family, Dentaliada, supplies the curious genus 

 Dtntalium, or tooth shell. 



The Patellida, or Limpets, constitute the third and a very 

 numerous family, distinguished at once by the form and structure of 



