MOLLUSC A. 185 



it is furnished with a complicated toothed organ or " odontophore " 

 {Oasteropoda, fig. 141, and Pteropoda) ; and lastly, the Cephalopoda, 

 in addition to an odontophore, possess horny mandibles, forming a 

 kind of beak, very like that of a parrot. 



The blood is colourless, or nearly so. In the lowest class of the 

 Mollusca {Polyzoa) the circulation is carried on by means of cilia, 

 and there is no distinct heart nor any definite course of the circulat- 

 ing fluid. In the Sea-squirts {Tunicata) there is a distinct heart, 

 but the structure of this is very simple, consisting of a mere tube, 

 open at both ends, so that the course of the circulation is periodically 

 reversed. In the higher Mollusca there is a distinct heart, consist- 

 ing of two chambers, of which one (the auricle) receives the aerated 

 blood from the gills, while the other (the ventricle) drives it through 

 the body. 



Respiration is very variously effected amongst the Mollusca. In 

 the Polyzoa (fig. 132) respiration is discharged mainly by the crown 

 of ciliated tentacles surrounding the mouth. In the Sea-squirts (fig. 

 134) respiration is effected by a greatly developed pharynx, which 

 is perforated by numerous ciliated apertures. In the Lamp-shells 

 and their allies {Brachiopoda, fig. 135) the long ciliated arms which 

 spring from the sides of the mouth seem to be the main agents in 

 respiration. In the Bivalve shell-fish, the Cuttle-fishes, and most 

 of the Univalves, the breathing-organs are in the form of gills or 

 branchise, adapted for breathing air dissolved in water. In the re- 

 mainder of the Univalves {e.g.. Snails and Slugs) the breathing- 

 organs are adapted for breathing air directly, and have the form of 

 an air-chamber or pulmonary sac, produced by the folding of a por- 

 tion of the mantle. The air is admitted to the chamber by a round 

 opening situated on the side of the neck, and capable of being 

 closed at will. The lining membrane of the chamber is richly sup- 

 plied with blood-vessels, and thus the necessary purification of the 

 blood is carried out. 



In accordance with the scattered or rudimentary condition of the 

 nervous system, the Mollusca are not characterised by acuteness 

 of senses, nor by any great power of locomotion. Organs of sight 

 exist in some of the lower and many of the higher Mollusca, attain- 

 ing in the Cuttle-fishes (fig. 148) an extremely high type of organi- 

 sation. Some of the common Bivalve shell-fish, such as the Scallop, 

 possess numerous simple eyes placed along the margins of the man- 

 tle, but in many cases even these are absent. Locomotion is very 

 variously effected, but seldom with much vigour or activity. The 

 lowest classes of the Mollusca are in the great majority of instances 

 fixed when adult. The common Univalve shell-fish, such as Whelks, 

 Snails, Slugs, &c., creejD about slowly by means of a flattened disc. 



