l86 AN INTRODUCTION TO THE STUDY OF FOSSILS 



tion, however, is independent of it, its function may be lymphatic 

 or more probably genital. 



The excretory organs consist of two large funnel-shaped 

 nephridia, one on each side of the intestine. They have a 

 wide inner opening into the ccelome and a narrower outer 

 opening near the mouth ; their function is to get rid of the nitrog- 

 enous waste gathered from the coelomic fluid. 



Respiration is effected mainly through the mantles, for the 

 blood sinuses branch broadly within them ; it is largely aided 

 also by the lophophore and its tentacles. 



There is a nerve ring around the oesophagus enlarged into a 

 dorsal and a ventral ganglion which give off nerves to the lopho- 

 phore, mantle, etc. No sense organs are known, but the animal 

 is sensitive to touch. 



The sexes are probably separate ; at least the same individ- 

 ual does not produce both ova and spermatozoa at the same 

 time. 



After the fertilization of an ovum by a spermatozoon the 

 single cell divides into two cells. Repeated division results in 

 the formation of a many-celled but hollow^ sphere, — the blas- 

 tula ; one side of this sinks in against the other, producing a two- 

 walled cup, the gastrula. After the cup-like opening of the 

 gastrula closes, the embryo becomes ciliated and is capable 

 of moving freely about. It now becomes gradually constricted 

 into three regions, — the head, thoracic and pedicle areas, 

 with a total length of about a third of a millimeter. From the 

 middle, or thoracic, segment grows a ring-like fold extending 

 back towards the pedicle segment. Soon, however, this cir- 

 cular fold or mantle divides into two, the dorsal and ventral 

 mantle lobes, which develop bundles of chitinous bristles upon 

 their free edges. After three or four days of this early embryonic 

 life, the animal settles down, attaching itself to some foreign 

 object by the end of the mucus-coated, sucker-like pedicle; 

 later the mantle lobes turning inside out, bend forward so as to 

 inclose the head segment and what is now the outer surfaces 



