GASTROPODA ISOPLEURA. 471 



and the common cord thus formed on each side unites with its fellow above 

 the anus. There are no pedal commissures, and the single ring surround- 

 ing the oesophagus appears to correspond with the buccal ring. The 

 nerve-cords contain ganglion-cells, as in Polyplacophora. Sensory organs 

 lodged in cavities of the shell-plates have been described in the last-named 

 order, and in some genera eyes as well. The latter may be exceedingly 

 numerous, and are furnished with a transparent cornea, pigmented envelope, 

 lens and retina. 



The mouth is more or less ventral in position. There is a muscular 

 pharynx. A radula is absent in Neomenia, present and furnished with very 

 complex transverse rows of teeth in Polyplacophora, with many teeth in 

 Proneomenia, and a single tooth in Chaetoderma. A remarkable sensory 

 sub-radular organ is found in Chiton. Small lobed buccal (salivary ?) glands 

 are present in Chiton, and a pair of long salivary tubes in Proneomenia. 

 Two saccular glands open into the oesophagus in Chiton (? all species). 

 Their epithelium undergoes remarkable colour changes during secretion, 

 and they are amylolytic, or salivary, in function. The intestine is straight in 

 Neomeniae and Chaetoderma ; ciliated in the former, and provided with 

 lateral liver caeca, with a single liver caecum in Chaetoderma. It is convo- 

 luted and ciliated in Polyplacophora, and there is a right and left liver 

 caecum, both lobulated and furnished with acini. The rectum opens on a 

 papilla in Polyplacophora, between the two branchial plumes in Chaetoderma, 

 and in union with the nephridial opening in Neomeniae. The heart is 

 situated posteriorly in a pericardial cavity. It consists of a ventricle and 

 right and left auricle in Polyplacophora : and, so far as is known, only of 

 a simple ventricle in other Isopleura. There is a dorsal aortic trunk run- 

 ning forward. There are apparently no specialised branchiae in Pro- 

 neomenia. They lie near the anus in the mantle cavity, and are tufted in 

 Neomenia, paired in Chaetoderma. In the Polyplacophora they have the 

 form of small ctenidia, arranged in the mantle furrow at the side of the 

 foot, and numbering sixteen or more on each side. The nephridia open 

 into the pericardial cavity. They have the form of short wide sacs with a 

 common external aperture in Neomeniae into the anus, or separate open- 

 ings, one on each side the anus, in Chaetoderma, and in these two orders 

 they serve as genital ducts. They are long, extend forwards, and are 

 folded upon themselves in Chiton, the folded portion bearing numerous 

 secretory acini ; and they open externally on each side in front of the 

 genital apertures and in the mantle furrow. The sexes are separate in 

 Polyplacophora and Chaetoderma, whereas the Neomeniae appear to be 

 hermaphrodite. The single ovary and testis are alike in Polyplacophora, 

 and are simple glands with a right and left duct opening posteriorly into 

 the mantle furrow. The hermaphrodite gland of Proneomenia appears to be 

 double. In it, in Neomenia and Chaetoderma, the genital duct or ducts open 



