G. A. DREW ON YOLDIA LIMATULA, SAY. 31 



SUMMARY. 

 A brief statement of the chief points is as follows : 



ANATOMY AND HABITS. 



1. The mantle has two pairs of sense organs, Fig. 1, ae, and pe, 

 one long unpaired tentacle, st, and a fringe of marginal tentacles. 

 All of these organs are sensitive to mechanical stimulation. The 

 unpaired tentacle seems to be homologous to one of the ordinary 

 marginal tentacles. 



2. The foot, Figs. 3 and 9, f, is a highly specialized burrowing 

 organ. 



3. The palp-appendages, Figs. 3, 10 and 18, pap, are food-collectors. 



4. The gills, Figs. 3, 19 and 20, are used in pumping water, for 

 which function they are well fitted by their structure. 



5. Each genital duct, Fig. 23, gd, joins the outer, not the inner, 

 end of the corresponding excretory organ, very near its opening into 

 the branchial chamber. 



6. No division into cerebral and pleural ganglia has been observed. 

 Neither do the cerebro-pedal commissures seem to have a double 

 origin. 



7. The otocysts are not connected with the surface by canals. 

 Canals, apparently in process of degeneration, Figs. 21 and 22, oc, are 

 present in adults. 



8. Both an anterior and a posterior aorta are present. 



EMBRYOLOGY. 



1. The original surface cells seem to give rise to the test, the 

 cerebral ganglia, the apical plate, and a new ectoderm inside the test. 



2. The cerebral ganglia, Figs. 46 and 47, are formed as invagin- 

 ations from the surface. 



3. The otocysts contain otoliths before the test is cast. Fig. 61. 



4. The test, the apical plate, the stalk connecting the test with 

 the anterior portion of the body, and the stomodreum from the 

 blastopore to the position of the definitive mouth, are all cast away. 



5. It seems that typical molluscan larvas may have been 

 developed from embryos resembling those of Yoldia, and that the 

 test of Yoldia and the velum of other forms are homologous. 



