THE ODONTOPHORA. 421 



may give the protruded extremity of the radula a licking mo- 

 tion, which is quite independent of the chain-saw action due 

 to the intrinsic muscles. 1 



The odontophore is developed very early, and it would be 

 interesting to know whether it exists in the young of those 

 few Odontophora in which it is wanting in the adult state. 



Salivary glands are very generally present in the Odonto- 

 phora? and the liver is usually large. 



As in the Mollusca in general, the blood-corpuscles are 

 colorless and nucleated. The blood plasma is red in Pla- 

 norbis. 



The heart may be wanting (Dentalium), or it may resemble 

 that of the Lamellibranchs in having two auricles ( Chiton, 

 Haliotis), and even in being perforated by the rectum (Ha- 

 liotls, Turbo, JVerlta); most commonly it consists of a single 

 auricle and a single ventricle. In the Cephalopods, it is hard 

 to say whether the two or four branchio-cardiac trunks which 

 open into the ventricle should be regarded as veins or as 

 auricles. An accessory " portal " heart has been described 

 in Doris. 3 Special respiratory organs may be wanting, their 

 place being taken by processes of the body, or by the walls 

 of the mantle cavity, or by the general surface. 



The branchiae, when present, are numerous lamellar pro- 

 cesses, or from one to four plume-like gills. Aerial respira- 

 tion is effected by the walls of a pulmonary sac, which is a 

 modification of the pallial cavity. 



The presence of renal organs, in the form of one or more 

 sacs situated close to the heart, open to the exterior on one 

 side, and, on the other, in relation, usually by means of a 

 glandular structure, with the returning current of blood, is 

 very general ; and, in many cases, these renal sacs communi- 



1 In my memoir " On the Morphology of the Cephalous Mollusca " (" Phil. 

 Trans.," 1852) I described the chain-saw action of the odontophore, as I ob- 

 served it in the transparent Flroloides and Atlanta, while living. But, as Tro- 

 schel has remarked in his excellent monograph ( ;t Das Gebiss der Schnecken," 

 erste Lieferung, pp. 19, 20, 1856), I did not sufficiently dwell on the frequency 

 and importance of the licking action produced by the extrinsic muscles. I am 

 still of opinion, however, that this action cannot be rightly described as a 

 movement of the radula following secondarily upon that of the cartilages, inas- 

 much as it is a motion of the whole odontophore. On the other hand, it may 

 be, as has been suggested to me by Mr. Geddes — who at my suggestion has 

 undertaken a reexamination of the structure of the odontophore — that the flex- 

 ure of the anterior ends of the odontophoral cartilages, by the intrinsic mus- 

 cles inserted into them, plays an important part in the motion of the radula. 



2 In Doliiim the salivary secretion contains free sulphuric acid. 



3 Hancock and Embleton, " On the Anatomy of Doris." (" Phil. Trans.," 

 1852.) 



