VII MOLLUSCA—THE PALLIAL COMPLEX 71 



nacreous layer. ^ Should this view prove correct, the Cephalopods would have to 

 be differently classified. The division into Tetrabrandda and Dihranchia would 

 have to disappear, as we cannot tell whether the fossil Ammonoidca were Tetra- 

 branchia, and are also ignorant as to when the Dihranchia developed from the Tetra- 

 branchia. The Cephalopods would then have to be divided into (1) Nautiloidea 

 with the extant genus Xcnitilus ; (2) Ammoiioidea with the still living Octopoda ; and 

 (3) Belemnoidea with the extant Dccapoda. 



Bivalve shelly plates called aptychi have been found sometimes in the last 

 chamber of the Ammonoidea, sometimes isolated. These have been proved to belong 

 to the bodies of certain species of Ammonoidea, and have been considered by some to 

 be protectives for the nidamental gland, by others as opercula, and by others again 

 as the analogues or homologues of the infundibular cartilage of the Dccapoda. No 

 one of these three views has as yet been generally accepted. 



V. Arrangement of the Organs in the Mantle Cavity and of 

 the Outlets of Inner Organs in that Cavity. 



A discussion of this subject at this stage will help to explain the asymmetry of 

 the Gastropoda and to simplify the discussion in later chapters. 



There are, in the mantle cavity, many important organs crowded together in a 

 comparatively small space, and into it also open all the apertures of the inner organs 

 except the oi'al aperture of the alimentary canal. The term " circum-anal complex," 

 though especially applicable to the arrangement in the Gastropoda, is not so suitable 

 as "pallial comjilex," which applies to nearly all Mollusca, and comprises not only 

 the pallial organs themselves, but the ajiertures of inner organs that lie in the 

 mantle cavity. 



The most important constituents of the pallial complex are the ctenidium, the 

 osphradium (Spengel's organ, olfactory organ, or accessory gill), the hypobranchial 

 gland, the anus, and frequently the rectum as well, the nephridial apertures and 

 often the renal organ also, the genital apertures, and frequently the pericardium, 

 with the enclosed heart. 



Starting with the Chitonidic, which, as has already been described (p. 42), 

 must be considered as the most primitive of all living Molluscs, we have :— 



The median anus, lying at the posterior end of the body in the mantle groove ; 

 on each side of it anteriorly the nephridial apertures, and again on each side, in 

 front of these, the genital apertures. 



Assuming this to be the primitive arrangement, we have the following important 

 variations. 



A. Gastropoda. 

 ]. Prosobranchia. 



('. Diotocardia. — In Fissurdla, the pallial complex is still (|uite symmetrical, 

 but instead of lying posteriorly, as in Chiton, it, together with the mantle and the 

 pallial cavity, lies on the front of the visceral dome. We have to imagine that the 

 whole complex has shifted forward along the right side of the body, so that the gill 

 originally on the left has come to lie on the right anteriorly, and that originally on 

 the right now lies anteriorly on the left, and the same applies to the other organs 

 belonging to the complex. 



1 Steinmann, Bericht Freiburg Gesellsch., iv. pp. 113-129. 



