MOLLUSOA. 165 



nerves to the great locomotive organ of most Mollusks, the 

 " foot," and is therefore called the " pedal " ganglion. The 

 third is known by the cumbrous name of the " parieto-splanch- 

 nic " ganglion, because it supplies nervous filaments to the 

 walls (parietes) of the body, and also to the internal organs 

 {splanchna). In all the higher Mollusks it is this scattered 

 condition of the nervous masses which distinguishes them so 

 sliarply from all other animals. Distinct respiratory organs 

 may or may not be present, and they may be adapted for 

 breathing air directly or through the medium of water. All 

 the higher Mollusca are simple animals, and perpetuate their 

 kind by means of the sexes, but many of the lower forms 

 have the power of producing colonies by continuous gemma- 

 tion, much as we have formerly seen in the Hydroid Zoo- 

 phytes. 



The digestive system in all the Mollusca consists of a 

 mouth, gullet, stomach, intestine, and anus, with the excep- 

 tion of a few forms in which the intestine ends blindly. In 

 some the mouth is surrounded by ciliated tentacles (Polyzoa, 

 Fig. 77) ; in others, it is furnished with two long ciliated arms 

 {Srachiopodd) ; in the bivalves (I/amellibranchiatd), it is 

 mostly furnished with four membranous processes or " palpi " 

 (Fig. 80, p) ; in others, it is furnished with a complicated 

 toothed organ or " odontophore " ( Gasteropoda, Fig. 83, and 

 Pteropoda) ; and lastly, the Gephalopoda,in addition to an 

 odontophore, possess horny mandibles, forming a kind of beak, 

 very like that of a parrot. 



The blood is colorless, 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 circulating 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, consisting 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 among the Mollusca. 

 In the Polyzoa (Fig. 7*7) respiration, is discharged mainly by 

 the crown of ciliated tentacles surrounding the mouth. In the 

 sea-squirts (Fig. 78), respiration is effected by a greatly-devel- 

 oped pharynx, which is perforated by numerous ciliated aper- 

 tures. In the lamp-shells and their allies {Brachiopoda), the 



