252 EMBRYOLOGY OF TEREBRATULINA. 



as to the anterior and posterior regions in the Brachiopoda. It is, in fact, now evident that the Brachiopods 

 are depressed animals, having an anterior or ventral, and a posterior or dorsal valve. MM. Agassiz and 

 Vogt are therefore wrong in regarding them as compressed animals like the Lainellibranchiate Mollusks ; that 

 is to say, as animals having a right and left valve. 



" The larva, moreover, can not only swim, but also creep. This latter mode of progression is effected by a 

 sort of rotation of the ventral valve alternating to the right and left. In this movement the animal pushes 

 by supporting itself principally upon the strongest of the bristles above mentioned." 



In Wiegmann's Archiv, Dr. Miiller communicates additional observations on the larval 

 Discina, a notice of which, follows from the Annals and Magazine of Natural History, 

 3d Series, Vol. vin., p. 505. 



" In the Annals for October, 1860, p. 310, we gave a short abstract of a description of the larva of a Brachio- 

 pod observed by Dr. F. Miiller at Santa Catharina, on the coast of Brazil ; he now adds some further details 

 from repeated observations in the summers of 1859 and 1860. The larva appears to occur late in the sum- 

 mer, from February to April. 



" When the little animals are placed in good-sized vessels with pure sea-water, they soon ascend slowly ; the 

 slightly gaping shells stand perpendicularly, the hinge-margin downward ; close to the anterior margin the 

 eight arms spread out horizontally like rays, with their tips slightly bent downwards ; and the roundish knob 

 situated between the uppermost pair projects beyond the plane of the arms. In this posture they move 

 slowly about near the surface. When strongly shaken, or sometimes without any perceptible reason, they 

 retract the arms and close the shells, which then slowly turn over and sink to the bottom with the free mar- 

 gin downwards. If the arms be again protruded, the hinge-margin also again turns downwards. 



" The duration of this state never exceeded five to six days, and in general the larvae adhered to the bottom 

 or sides of the vessel in a still shorter time. When they adhered to the sides, the mouth was always directed 

 downwards ; the ventral shell was strongly drawn forward until its anterior margin reached or passed that of 

 the dorsal shell ; the transversely oval plate, previously concealed within the shells (the peduncle), was pro- 

 truded, apparently twisting around the notched hinder margin of the ventral shell, so that its anterior margin 

 became posterior. For a day or more the animal remains contracted and quiet; then, the shells being 

 slightly opened, the arms are half extended, and strike inwards, one or more at a time, just as in the marine 

 Bryozoa. In a few days new bristles appear at the anterior margin, in the space left between the more deli- 

 cate setae of the dorsal shell. In a week the author counted twenty of these, mostly belonging to the dorsal 

 shell. The longest were 0.8 mill, in length, straight, colorless, 0.006 mill, in thickness at the base, terminat- 

 ing in a fine point, unjointed and distantly feathered with fine lateral setae 0.02 mill, in length. The principal 

 change in the soft parts consisted in the retrogression of the organs of sense. The eyes had become broken up 

 into groups of about ten black points ; the previously spherical auditory vescicles were shrunken into longish 

 Bfics, closely surrounding the otoliths. In somewhat older animals there was no trace of the organs of sense, 

 although they had not lost their sensibility to light. 



" One of the larvae lived for a month after its adhesion ; but from the lapse of a day before its death was 

 noticed, the soft parts had become greatly decomposed. The older bristles of the free larvae appeared to be 

 still present, as also the plumose bristles of the anterior margin. Besides these, there were on each side, about 

 in the middle between the median line and the origin of the great bristles of the fourth pair, straight, smooth 

 bristles, 0.2 mill, in length, projecting obliquely backward, little thicker than the strong posterior setae, but 

 with a much stronger outline. 



"It is remarkable that in two years the author has repeatedly captured free-swimming larvae which had 

 evidently advanced further in their development than the oldest of those which had already fixed themselves. 

 They were all destitute of the transversely oval plate, and of every trace of organs of sense; the plumose 

 setae of the anterior margin were also wanting, as were, more or less entirely, the older bristles. Of the 

 more delicate bent bristles, some were usually still present, and these appeared to be unabbreviated, so that 

 the missing ones had probably been lost by shedding. The stronger bristles, on the contrary, are gradually 

 absorbed at the base ; at lenst this is the case with the fourth pair ; these were repeatedly met with of about 

 half their proper length ; the stalk, with its fusiform dilatations, had disappeared, while the apex remained 

 readily recogu'z.'.ble by its peculiar curvature and denticulation. In a still older animal about a fifth of the 

 length was still present, so that it no longer extended beyond the margin of the shell. This animal (the 

 oldest ex iiuined by the author) had lost all the older setse, except the small residue. On the other hand, the 

 two straight, smooth bristles, which in the oldest attached animals scarcely began to protrude from the shell, 



