vii THE HYPOTHETIGAL PRIMITIVE MOLLUSC 27 



e.g. the dorsal fold or carapace which, in the higher Crustacea, covers 

 the branchial cavity, and the operculum of Fishes. The relations 

 existing between the branchiae, the mantle, and the shell in the 

 Mollusca are of the highest importance ; these organs should always 

 be regarded as essentially interdependent structures. 



The branchiae lying in the mantle cavity are paired and symme- 

 trical. It may be left an open question whether the primitive Mollusc 

 possessed more than one pair of gills. If only one, we must suppose 

 that one gill lay on each side of the mantle cavity posteriorly ; if more 

 than one, that there was a row of branchia3 on each side. 



Each gill is feather-like, with a shaft and two rows of very 

 numerous leaflets. The shaft stands out freely from the body in the 

 mantle cavity. Close to the base of each gill, a sensory organ, con- 

 sidered to be olfactory, and called the osphradium, is found. Such 

 a gill with an osphradium at its base has a very definite morphological 

 value; in order to distinguish it from analogous though not homologous 

 respiratory organs found in certain Mollusca, it has been named a 

 etenidium. 



The head is provided with one pair of tentacles and one pair of 

 eyes. The mouth lies anteriorly and ventrally. The remaining open- 

 ings of the inner organs lie posteriorly above the foot ; the anus in the 

 middle line, and on each side, between it and the etenidium (supposing 

 that there is only one pair of ctenidia), an aperture for the sexual 

 organs, and another for the kidney (nephridium). These five apertures 

 are covered by the mantle, and thus lie in the mantle cavity. We 

 have thus, to recapitulate, in the posterior part of the mantle cavity 

 two ctenidia, two osphradia, and five apertures, the median anus, and 

 the paired symmetrical sexual and renal apertures. These, taken 

 together, form what is known as the pallial complex. 



The inner organisation may thus be briefly described. 



The intestinal canal. The mouth leads to a muscular pharynx, with 

 horny jaws. At its base lies a chitinous rasp-like ribbon called the 

 tongue or radula, which carries numerous consecutive transverse rows 

 of sharp chitinous teeth. Paired salivary glands enter the pharynx, 

 which passes into an oesophagus, which latter leads into the mid- 

 gut. This, which we will suppose to be more or less coiled, runs 

 right through the body, passing posteriorly into a very short hind- 

 gut, which opens outward through the median anus. The mid-gut 

 has large paired glandular divertieula (mesenteric gland, diges- 

 tive gland, hepatopancreas, liver). 



Musculature. The muscles of the foot are powerful, and are 

 adapted for the creeping movement. There are, in addition, muscles 

 running from the inner surface of the shell into the foot and head 

 (eolumellar or shell muscles), and special muscles for the different 

 organs. 



Nervous system. Two well - developed cerebral ganglia lie 

 dorsally in the head, and are connected by means of a short cerebral 



