vii MOLLUSGATHE PALLIAL COMPLEX 81 



C. Lamellibranchia. 



The general arrangement of the organs in the mantle cavity of the Lamelli- 

 branchia has already been described. The strict symmetry of the body in this 

 class must again be pointed out. All originally paired organs remain paired and 

 symmetrical. 



The two nephridial apertures lie on the body above the base of the foot, or 

 farther back near the posterior adductor muscle ; they usually lie beneath the point 

 of attachment of the gill-axis, between it and the line of concrescence of the (inner) 

 ascending lamella of the branchial leaf with the foot, where such concrescence takes 

 place. In the Scptibranchia, on the contrary, the apertures open into the upper 

 pallial chamber. 



The outer genital apertures may be wanting, and in this case the genital 

 products are ejected through the nephridial apertures, which is the primitive 

 arrangement. When present, in diceceous bivalves, they are always found in one 

 pair, and lie on each side just in front of the nephridial apertures, sometimes in the 

 base of a common pit or furrow, less frequently at some distance from these aper- 

 tures. There are no special copulatory organs. 



In hermaphrodite Lamellibranchia the arrangements may vary as follows : 



1. Both kinds of sexual products may be ejected on each side through a 

 common aperture (Ostrcca, Pectcn, Cydas, Pisidium, etc.). 



2. There may be, on each side, two distinct apertures, one male and the other 

 female (Anatinacea). 



3. The seminal ducts and the oviducts may unite before opening to form a 

 short, common, terminal piece (Septibranchia). 



The osphradium is paired in the Lamellibranchia, and always lies near the 

 posterior adductor muscle over the visceral ganglion, at the point of insertion of the 

 branchial axis on the body. A pair of sensory organs is found in many Lamelli- 

 branchia, one on each side of the anus (abdominal sensory organs), or to the right 

 and left on the mantle at the inner aperture of the siphons of the Siphoniata (pallial 

 sensory organs). 



Hypobranchial glands have been found in the Protobranchia (Nuculidcc and 

 Solenoiiujidce}. They are large and well developed, and belong to the mantle, lying 

 in the posterior part of the body above the base of the gill on each side, to the right 

 and left of the pericardium, and in front of the posterior adductor. 



The leaf-like oral lobes (labial palps), one occurring on each side of the mouth, 

 between it and the anterior end of the base of the gill, will be described more in 

 detail in another place. 



D. Cephalopoda. 



In the Cephalopoda the primitive symmetry of the pallial complex is on the 

 whole retained. 



If we cut open the mantle of the Nautilus (Figs. 78 and 79), which covers the 

 posteriorly placed pallial cavity, and lay it back on all sides, the following organs 

 are revealed : 



1. On each side there are two gills, an upper and a lower. 



2. The anus lies on the visceral dome, between the bases of the four gills. 



3. Below the base of each gill is found a nephridial aperture making four in all. 



i. Close to the two upper nephridial apertures lie the two so-called viscero- 

 pericardial apertures. 



5. Between the bases of the lower gills there are in each sex, two genital 

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