84 COMPARATIVE ANATOMY CHAP. 



apertures, but only that on the right side is functional. In the male, the aperture 

 is produced into a tubular penis. 



6. Above the bases of the lower gills there is an osph radium on each side placed 

 on a papilla. 



7. Above the anus there is a large median papilla of unknown significance. 



8. The nidamental gland lies dorsally in the mantle. 



If we compare with the above the pallial complex of a dibranchiate Cephalopod, 

 such as Sepia (Fig. 80), we find the following arrangements : 



1. There is one gill on each side. 



2. Along the median line of the visceral dome, the rectum and the duct of the 

 ink-bag descend together, to open through a common aperture at the tip of a papilla 

 at the base of the siphon. 



3. On each side near the rectum, above the anus, a nephridial aperture occurs 

 on the point of a papilla. 



4. Of the two paired genital apertures only the left has been retained in Sepia 

 and many other Cephalopods ; this lies near the left nephridial aperture at the 

 summit of a large papilla (penis). In the female Octopit*, the genital apertures are 

 paired and symmetrical, and lie to the right and left of the rectum. 



5. The two nidamental glands (in D<'c<ij>i/a) lie in the visceral dome, sym- 

 metrically with regard to the median line ; they open above the nephridial apertures 

 into the mantle cavity. 



VI. The Respiratory Organs. 

 The True Gills or Ctenidia. 



The most important of the pallial organs in the Mollusca is the gill, 

 for it is in order to protect it that the mantle, and with it the pallial 

 cavity, develop. The gill found in the mantle cavity is throughout 

 all the divisions of the Mollusca a homologous organ, to be derived 

 from the gill of a common racial form. But since this gill is wanting 

 in certain Mollusca (e.g. many Opisthobranchia), and is functionally 

 replaced by new organs which are morphologically altogether uncon- 

 nected with it, it has been found useful to distinguish the primitive 

 Molluscan gill by the name of etenidium. This word, therefore, has 

 a special morphological significance. 



The etenidia of the Mollusca are originally paired and symmetri- 

 cally arranged ciliated processes of the body wall, carrying two 

 rows of branchial leaflets, and projecting into the mantle cavity. 



Venous blood flows into the gills through afferent vessels 

 (branchial arteries), and after becoming arterial by means of the 

 respiration, flows through efferent vessels (branchial veins) back to 

 the body, passing first through the heart. At or near the base of 

 each etenidium there always lies a sensory organ, which is considered 

 as olfactory, the so-called osphradium or Spengel's organ. 



Such primitive etenidia are met with first in that group of the 

 Mollusca which has undoubtedly retained more primitive characteristics 

 than any other, viz. the Chitonidce among the Amphineura. They are, 

 further, found in all other Mollusca which have retained the original 



