EMBRYOLOGY OF TEREBRATULINA. " 253 



had attained a length of double the diameter of the shell, and being inserted into thick muscular slieaths 

 were strongly and vapidly moved by the animal, sometimes spread' out horizontally, sometimes again crossed 

 backwards. 



'■ During this complete change of the setiE the soft parts bad umlergone no essential alterations. The 

 roundish stomach, reaching from, the front to the middle of the longitudinal diameter, still showed the two 

 dark spots of the young larva;, which remind one of the similar spots in the larvoe of some Bryozoa. From 

 the back of the stomach spi-ang the intestine, which bends under the margin of the stomach to the right, and 

 then forwards, terminating about the middle of its right side. The oesophagus goes from the front of the 

 stomach straight forward half way to the front of the shell, and then bends downward, so that the mouth lies 

 close to the stomach. The arms, especially the two middle pair, had become longer and slenderer, and the 

 knob between the antei-ior pair had diminished in size. No vessels or pulsating heart were recognized. — 

 Wiegmann's Archiv, p. 53, 18G1. " 



In the year 1869 I communicated to the American Association for the Advancement of 

 Science the results of a study of the early stages of TerehratuVuia septentrlonalls, made at 

 Ea,stport, Me., in the early part of that year. This was afterward published in the Memoirs 

 of the Boston Society of Natural History, accompanied by two plates. 



In this memoir I showed the incipient cirri, seven or eight in number, and the gradual 

 development of the complicated arms, the unfolding of the hepatic coeca from two folds 

 upon the walls of the digestive cavity, the character of the shell structure, etc. And these 

 two papers comprised all that has yet been done as to the early stages of the class. 



A wide gap, however, has existed between the free swimming Annelidan larva clothed 

 with cilia and the attached form reveahng its Brachiopodan character in the j^resence of a 

 dorsal and ventral plate, and the presence of cirri. After repeated visits to Eastport, al- 

 most solely for this purpose, I have at last succeeded in closing this gap, and in this paper I 

 hope to make plain the history of the development of the dorsal and ventral areas, the 

 peduncular attachment, and the relations the different parts of the mature animal bear to 

 the embryonic segments, as well as to present some new features in the early stages of 

 the species. 



For two seasons I have found Terebratulina spawning at Eastport during the last of May 

 and the early part of June. It is probable that they spawn through the season, since I 

 have found them depositing eggs in April. At my request Mr. Rathbun made several 

 observations during the summer, to ascertain how long the species continues to ovulate, and 

 he informs me that on the onl}^ dates he collected them, namely, June 26, July 12 and 

 Aug. 29, they were freely discharging their eggs. It is best, however, to study them as 

 early in the season as possible, as the water in which the eggs develope must be kept at a 

 low temperature, and this essential condition can be most easily accomplished at Eastport 

 in May, where the temperature at that time ranges from forty to sixty degrees Fahrenheit. 



The manner in which I secured the eggs for study was simple enough. Several shalloAv 

 glass dishes were prepared by painting the bottoms black. Having dredged a lot of speci- 

 mens, I arranged them in a circle around the edges of the dish, with their anterior ends 

 pointing toward the centre of the dish. By arranging the specimens in this manner the 

 difference in the sexes becomes at once apparent. 



The eggs are discharged from the anterior margm and drop just lie^^ond the pallial mem- 

 brane, hancrine: in clusters from the setce, and coverino; the bottom of the dish in the imme- 

 diate vicinity of the animal, presenting the appearance of a white powder, though with a 

 simple lens the individual eggs are plainly seen. They are opaque and spermaceti white in 



MEMOlIlb BOST. SOC. NAT. HIST. VOL. 11 



