MOLLUSCA 499 



arise from the dorsal side of the head and serve partly as sense organs 

 and partly for seizing the food. The foot is conical and can be pro- 

 truded for use as a digging organ. 



There is a well-developed radula, a mantle, which in the larva is 

 produced into two lobes (which fuse later), a nervous system with 

 separate cerebral and pleural ganglia and a symmetrical visceral loop. 

 The kidneys are paired; they do not have an opening into the 

 perivisceral coelom. These characters, with the exception of the first 

 and last, bring the Scaphopoda near to the primitive lamellib ranch. 

 In the two following morphological features the group is so specialized 

 that it stands apart from any other division of the Mollusca. 



There are no ctenidia, respiration taking place by means of the 

 mantle. The circulatory system is remarkably simplified and there is 

 no distinct heart. 



The gonad discharges into the right kidney as in the Diotocardia 

 among Gasteropoda. 



Class LAMELLIBRANCHIATA 



Mollusca in which typically the body is bilaterally symmetrical, much 

 compressed from side to side and completely enveloped by the mantle 

 which is divided into two equal lobes ; each lobe secretes a shell valve, 

 the two valves being joined dorsally by a ligament and hinge and closed 

 ventrally by the contraction of one or two transverse adductor muscles \ 

 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 burrowing ; 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. 367) 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, htconnng 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 



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