EMBRYOLOGY OF TEREBRATULHSTA. 257 



contracted. In many embryos the thoracic ring presents an indentation on one side, as if 

 the ring were a band partially nniting at this place. The peduncle gradually widens at the 

 end into a sucking disk, and at this time the emljryo becomes permanently attached. 



Fourth Stage. The embryo now settles down upon its base of attachment, the tho- 

 racic segment widening and flattening, and appearing pressed to its point of support, so as 

 to conceal the jjeduncle. The head is closely pressed to the thoracic segment, the clear 

 space within having disappeared, and tlie head only appearing ciliated. And now com- 

 mences the most important portion of the developmental history, in which, for the first 

 time, Ave l^ecome acquainted with the development of the dorsal and ventral plates. 



Fifth Stage. The thoracic segment commences to fold upon both sides of the head, and 

 these are the dorsal and ventral folds. Gradually the head is enclosed by the folds. In this 

 portion of their history I was peculiarly fortunate in securing hundreds of examples just 

 after they had become attached to adult Terebratulina. These I studied in their natural 

 positions. In some cases the Terebratulina had been torn from Mytilus, to which they had 

 been attached. The peduncular disk brought away a layer of the black epidermis of the 

 Mytilus, and to this surface many embryos conveniently became attached, and agamst this 

 black background the pure white embryo showed to the best advantage. At intervals the 

 Terebratulina would twirl its peduncle, thus presenting the embryos in different positions, all 

 of which I instantly sketched. I have a great many drawings at every stage of this 

 interesting growth, some of which are given on Plate viii., Figs. 47 to 77, inclusive. 



One fold grows more rapidly than the other, and I infer that this is to be the larger or 

 ventral plate. The head appears to be gradually engulfed, as it were, though for a long 

 time protruding beyond the dorsal and ventral folds. The folds also present for a long 

 time a thickened and rounded border. The inner edge of what I suppose to be the dorsal 

 valve, shows a distinct notch in the median line in some (Plate viir.. Fig. Go). As the dor- 

 sal and ventral folds develop, the embryo becomes elevated upon the peduncular stalk, and 

 the attachment becomes so firm that a forcible stream of water falling upon them does not 

 detach them, indeed a point of a needle, or a stiff camel's hair pencil, was found necessary 

 for the purpose, and in the removal, the parts were so soft as to break down in most cases. 

 The mouth also makes its appearance at this time, the head for a long time occupying the 

 area enclosed by the two folds. As soon as the embryo rises upon its peduncle it drops 

 partially over to one side, the ventral or larger area being uppermost. 



The hinge margin is broad and sliglitly rounded when looked at from above ; a side view, 

 however, presents a wide and flattened area, as is shown in some species of Spirifer, and 

 the embryo for a long time assumes the position that the Spirifer must have assumed. The 

 dorsal and ventral folds close up on the sides, and their edges become sharper ; the future 

 lateral openings of the shell gradually extend to the base. From above, the thorax is wider 

 than long. Starting from a broad base it widens gradually to the anterior margin, which is 

 nearly straight, to which it joins by a short curve. A heart-shaped corneous shell is foinned 

 even at this early stage, for in several cases I met Avith it where the softer portions had 

 been removed by Paramsecia. (See Plate viii.. Fig. 68.) 



Before the folds have closed over the head, four bundles of setoe appear ; one bundle 

 containing from seven to nine setas springing from Avhere the folds unite laterally. These 

 stand out at nearly right angles to the longitudinal axis. The other bundles containing 

 from two to fom' setiB, spring from a point midway between the first bundles and the 



MEMOIRS BOBT. 800. NAT, HIST. VOL. II. 65 



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