MOLLT7SCA. 163 



branchia forming the second family, Monopleurobrandiiata (or 

 Pomatobranchiata). 



Phyllidiidfs. Pleurophyllidiidce. 



Phyllidia. Pleurophy llidia = Diphyllidia. 



TECTIBRANCIIIATA ( = Pleurobranchia). Branchiae feather- 

 shaped, on the right, rarely on both sides. A shell in most, en- 

 closed in the mantle. 



Branchiae lie under the edge of the mantle. Shell mostly in- 

 ternal, unsymmetrical, either discoidal, subspiral, or involute 

 with one or more whorls. 



Runcinida. Lophoeercus=0xy- Tornatettida. 



Order HI. PROSOBRANCHIATA. 



Eespiration aquatic, effected by pectinate or plumose branchiae 

 placed in a cavity in front of the heart. Animal enclosed in a 

 univalve shell [multivalve in Ghitonidse]. Dioecious. Larva with 

 a velum and shell. 



The shell is sometimes simple, but most commonly it is spiral ; 

 in the latter the mouth [peritreme] is variously shaped, and the 

 whorls coil round a pillar [coluruella], frequently perforated by a 

 canal [umbilicus]. The animal is always capable of withdraw- 

 ing entirely within the shell, which is sometimes closed by an 

 operculum. The mantle is placed over the back of the head, and 

 the branchiae are lodged beneath it, anterior to the heart. Loco- 

 motion is invariably, except in lanthina, performed by crawling. 



To this order belong the limpet (Patella vulgata), money-cowry 

 (Cyprcea moneta\ whelk (Buccinum undatum}, and periwinkle 

 (Littorina lift or ea). Various species of Purpura and Murex 

 yielded the " purple " of the ancients. 



lanthina, now generally referred to this order, is provided with 

 a float which resembles a mass of soap-bubbles, having no organic 

 connexion with the animal, and to which the female parent commits 

 its ova, attached to one extremity. The float is then detached, 



M2 



