518 Causes and Course of Organic Evolution 



In tracing the class backward from the present time, zool- 

 ogists have recognized that gradual simplification can be noted 

 from living and also extinct members of the division Testi- 

 cardines to species and genera of the much simpler Ecardines, 

 the latter of which are found fossil in the most ancient Cam- 

 brian rocks. Throughout this entire history also, all were 

 marine, and secreted at first a chitinous or semicalcareous 

 shell that eventually became modified into the huge heavy 

 calcareous shells of examples like Productus giganieiis that 

 the writer as a boy laid bare abundantly in rocks of the car- 

 boniferous age. So, in any attempt that may be made now 

 to trace their phylogeny, we must trust to embryonic resem- 

 blances, or to picturing their developmental trend during 

 archsean times by interpreting more primitive stages in light 

 of the changes passed through from Cambrian times till now. 

 The latter seems to avail little, the former therefore must 

 be the mode of attack. 



Though our knowledge of brachiopod embryology is still 

 very imperfect, enough has been published to act as a fair 

 guide. As in adult rotifers the young embryo of Cistula or 

 Argiope shows division into three regions, the head, the body, 

 and the foot. Similarly the head region, which in structure 

 and relation can exactly be likened to the trochal disc, is cili- 

 ated and bears a circlet of cilia round the margin — the prseoral 

 ring. In exact position as amongst some rotifers, four simple 

 red or brownish eyes are formed. From the position of these 

 in relation to the ganglia of the adult, the ganglia seem to 

 arise between the eyes which are doubtless connected with 

 them. The body region is deeply constricted off from the 

 head, much after the manner of Megalotrocha amongst rotifers. 

 Further the upper circular rim of the body grows outward 

 and downward as a skirt around the body, and this bears four 

 patches of setae. Alike the outgrowth and its setae correspond 

 well to the setigerous lobes of Pedalion amongst rotifers, while 

 in both the setae are movable and can be expanded. The 

 simple alimentary canal opens by a mouth under the prseoral 

 ciliated ring. The stalk or foot is as in rotifers a tapered cone, 

 the apex of which is naturally the future attaching or pedal 

 center. From its relation to the adult animal it Seems prob- 

 able that pedal cement glands are present here as in rotifers 

 and polyzoans, for "after swimming about for a few hours, 

 the larva fixes itself finally, apparently adhering by some se- 

 cretion produced by the stalk" (or foot) "segment" (170: 482). 



The free-swimming brachiopod larva mo^es exactly as does 

 the rotiferan, for it keeps the ciliated highly sensitive head 



