238 



HIGH SCHOOL ZOOLOGY. 



if 



are secreted by mantle-lobes, whicli, of course, have likewise a 



totally different relation to the body 



from what exists in the Lamellibranchs. 



Their edges are beset with setae, like 



those of worms, and they contain pro. 



cesses from the coelom in which the 

 Fijr. \&X—Rhvnchonella plena. , i x- i rn, • i • 



Chazy Formation. eggS are to be tound. iho animal IS 



confined to the attached end of the shell (Fig. 164) the roomy 

 mantle-cavity being 

 occupied by a pair of 

 coiled-up arms, fring- 

 ed with tentacles, 

 which take their ori- 

 <rin on either side of , 

 the mouth. These 

 arms (at one time 

 supposed to be equiv- 

 alent to the Molluscan Fig. 164.— Diagrram of Rhynchonella, aa seen from the 



. , side with the shell jiartly removed. 



foot hence the name «, anterior ; h, posterior or hinge area of shell ; p. 



„ , •, V _ peduncle, s, stomach embedded in the liver and com- 



OI the class — J SJ-re municating with the cesophag-us in front,,and the intestine 



-, ill behind : d, opening, o, closing muscles. 



often supported by a 



calcareous endoskeleton ; such is not the case in Rhynchonella, 

 but in some of the fossil forms (Fig. 16.5), the complete skeleton 



is well preserved. It is obvious, 

 then, that the arms are not for 

 locomotion; their chief function 

 is to bring food to the mouth, and 

 this is effected by cilia on the 

 tentacles, which create a current 



Fig. 165.-Intemal surface of dorsal valve down the Coils to the mouth. 

 of a Spirifcr. q^q q^. ^^q pairs of nephridia 



are present, more nearly resembling those of Vermes than of 

 Mollusca. The intestine ends blirid in Rhynchonella and other 

 forms which have a hinge to the shell (Testicardines), but in 



