THE SCAPIIOPODA. 431 



may be merely cliitinous or completely calcified, or partly in 

 the one and partly in the other condition, are developed in 

 sacs lined by the cells of the ectoderm.' In the pallial groove 

 lie the short lamellar processes which represent the branchiae. 

 The shell is unlike that of any other Mollusk. It consists of 

 eight, transversely elongated, synuuetrical pieces, arranged 

 one behind the other, overlapping in yuch a manner that the 

 posterior edge of the one covers the anterior edge of the 

 next, and articulated together. Sometimes the valves arc 

 partially or completely inclosed in the mantle. The heart, 

 composed of a single median ventricle and two lateral auri- 

 cles, is placed in the middle line, above the rectum, at the 

 posterior end of the bodv. The aorta is continued forward 

 from its anterior end, while the auricles receive the blood 

 from the branchijxi. In Chiton plceus, according to Schiff,* 

 each auricle conmiunicates by two openings with the ven- 

 tricle, and the two auricles are united behind. The repro- 

 ductive organ is median and symmetrical, and its two ducts 

 open on each side of, and not far from, the anus. 



The embryo leaves the cgfr as an oval body, surrounded 

 near its anterior end by a circular ciliated band, behind which 

 an eye-spot appears on each side (Fig. 121, III.). Tlie seg- 

 ments of the shell appear while the young Chiton is still 

 locomotive, and the disk in front of the ciliated band becomes 

 converted into the lobe above the mouth (Fig. 121, IV., V.). 

 The Chitons have existed from the Silurian epoch to the 

 present day, appnrently with very little modification. 



The Scapiiopoda.^ — In J)eiit(flif()n^ the shell is elongated, 

 conical, and curved, like an elephant's tusk, with the apex 

 broken off, and it is open at both ends. The animal has a 

 large mantle corresj-jonding in form with the shell, and also 

 open at both ends, the margins of the anterior, larger, aper- 

 ture being much thickened. The month, |)laced at the extrem- 

 ity of a sort of cup, the margin of which is fringed with pa- 

 pilhie, is situated far behind the {interior opening of the man- 

 tle. Behind the oral cup, where the body joins the mantle, 

 is a transverse muscular ridge, from which proceed a great 



1 Rcincke, "Beitr;icre zur Rildiin^sgcscliiclitc dcr Staclicln, u. s. w." (Zeit- 

 schrift fiir wii^-fenscI/afflirJic Z'lo/tv/ic.) 



2 Zeitschriftfur wlss(>ii!^i'}inftlirhe Zoologie, 1858. 



3 A very complete and aeeiirate account of the orjjanization of Dcntalium is 

 priven in the monoffraph of Lacaze-Duthiers, " Tlistoire de I'overanisation, du 

 ^eveloppement, des moeurs et des rapports zoologiqucs dcs Dentales," 1858. 



28 



