2GO EMBRYOLOGY OF TEREBRATULINA. 



the body into segments, and their power of contracting upon each other is concerned, 

 though the conditions under which they develop will be found to be entirely unlike. The 

 egg of Terebratulina is discarded freely into the water to undergo its development, and 

 possibly its fertilization outside the parent, while the embryo Thecidium is held attached 

 within the pallial cavity, and separates only after it has undergone changes in advance of 

 those found in the embryo Terebratulina after it has become permanently attached. 



The presence of eye spots in the embryo Thecidium is another important difference, for 

 though I examined hundreds of the Terebratulina embryos, yet in no instance were the 

 traces of eye spots detected. On comparing the early stages of Terebratulina with that of 

 Discina, we are at once struck by the marked differences between the two ; Terebratulina 

 becoming permanently attached by its peduncular segment long before a trace of the pecu- 

 liar dorsal and ventral plates makes its appearance, or even before any definite structure 

 shows within, while Discina swims freely in the water sometime after the dorsal and ven- 

 tral plates, cirri, mouth, oesophagus and stomach, have made their appearance. 



The long and protrusible oesophagus and head, bearing a crown of eight cirri, or tenta- 

 cles, in Discina, as described by Dr. M tiller, is unlike anything of the kind observed in 

 Terebratulina. Their barbed and deciduous setas, however, present similar features to those 

 of the embryo Terebratulina. 



The embryology of Lingula (which I hope soon to have the opportunity of studying on 

 the coast of North Carolina) will undoubtedly afford stages similar to those observed in Dis- 

 cina. I have already studied, in the transparent plates of Lingula pyramidata, the charac- 

 ter of the nucleus, and find it perfectly orbicular with a margin finely notched, and in cer- 

 tain fossil LingulaD I have observed the same orbicular nucleus. With the other characters, 

 in common between Lingula and Discina, we should expect to find similar features revealed 

 in their embryology. 



In considering the various degrees of persistence of embryonic features in the few forms 

 we are thus far acquainted with, we are struck with the great difference in this respect be- 

 tween Discina on the one hand, and Terebratulina on the other. Thus, in Discina, attach- 

 ment takes place sometime after adult characters make their appearance, the peduncle at 

 first extending directly backward, as in Lingula. In Lingula, as I have heretofore ob- 

 served, attachment never takes place, at least in L. pyramidata, the creature living loose in 

 the sand ; and this feature will probably be found characteristic of other LingulaB when 

 they shall have been carefully observed. 



Thus we see in later geological forms, attachment taking place earlier in developmental 

 history than in those of earlier geological times. Similarly in earlier geological times we 

 find forms in which the dorsal and ventral plates are chitinous, and, as I have observed in 

 Lingula pyramidata, of such transparency that the circulating fluid could be easily seen 

 coursing through the sinuses of the pallial membranes, and in this latter species containing 

 so little earthy matter that w r hen dry they become twisted out of all shape, and even roll 

 up like a leaf. With these facts it requires no hazardous supposition to conceive the pri- 

 mordial Brachiopods devoid of the dorsal and ventral plates, or furnished only with a semi- 

 lunar membrane on the head, as in that curious annelid Umbellisyllis, described by Sars, 

 and of a form that presented the annelidan characters less disguised by features that have 

 heretofore prevented a right conception of their affinities. 



As to the relations of the Brachiopods with the Polyzoa, some features of similarity are 



