574 THE INVERTEBRATA 



the head is rudimentary, eyes, tentacles and radula being absent ; there 

 is a pair of labial palps with the mouth situated between them ; the 

 foot is ventral, without a crawling surface but usually wedge-shaped 

 and adapted for progression in mud or sand ; there are two ctenidia 

 in the mantle cavity, often greatly enlarged and with a complicated 

 structure; their cilia, together with those of the labial palps, form 

 a mechanism for the collection of small food particles ; the sexes are 

 nearly always separate, and there is a trochosphere and a veliger larva 

 in the marine forms. 



The development of the ctenidia (Fig. 392) is the outstanding 

 morphological and physiological character of the lamellibranchs. The 

 arrangement of the shell valves, which allows the mantle cavity to 

 extend the whole length of the body, also makes possible a great ex- 

 tension of the ctenidia. The axis increases in length and the branches 

 on each side not only increase in length, h&covmng filaments ^ but also 

 turn up at the ends so that there is a descending and an ascending limb. 

 The limbs of adjacent filaments are connected together by ciliary 

 junctions (Mytilus), or by growth of tissue (Anodonta), so that thus all 

 the filaments are joined together to form gill plates, each gill plate 

 consisting of two lamellae formed from all the ascending and all the 

 descending limbs respectively. The lamellae are united by cords of 

 tissue which constitute the interlamellar concrescences. The extent to 

 which the gills are welded together to form continuous plates is the 

 distinction between the three main groups of the Lamellibranchiata, 

 the Protobranchiata [Nucula), the Filibranchiata [Mytilus) and the 

 Eulamellibranchiata (Anodonta). But even in the last-named group 

 there are left occasional holes through which water passes into the 

 interlamellar spaces then into the epibranchial space dorsal to the gills. 



Belonging to the same physiological system are the labial palps, two 

 folds, one in front of the mouth and one behind, which are turned 

 backwards and prolonged on each side of the visceral mass so as to 

 form two pairs of richly ciliated triangular flaps, embracing the 

 anterior end of the ctenidia, and enclosing a groove which leads to 

 the mouth. 



In the anterior part of the mantle cavity the axis of the gill is at- 

 tached to the side of the animal dorsal to the foot, which here forms 

 a vertical partition dividing the cavity into a right and left half. The 

 mantle cavity continues behind the foot, however, and here the up- 

 turned ends of the inner rows of filaments of both ctenidia are united 

 so that the mantle cavity is now divided by a horizontal partition into 

 an upper or epibranchial cavity and a lower main cavity. The former 

 opens at the dorsal siphon, the latter at the ventral siphon. A constant 

 current of water is maintained during activity, entering by the ventral 

 siphon, passing through the gill lamellae, and leaving by the dorsal. 



