414 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



the latter lias a different inuervation, while the miiseular septuiQ proper is inuervated 

 by the branchial nerve, and that it is in direct commnnication with the efferent lacnnse 

 of the auricles. 



DETAILED STRUCTURE OF THE GILLS. 



The gills, so extremely varied in structure, are of very great importance on account 

 of the relationship which they show to exist among various forms of the group, and 

 have been thoroughly studied in a great many genera. The work of Posner (No. 19), 

 Peck (No. 16), and Mitsukuri (No. 13), extended over a great field, and, together with 

 the embryological observations of Loven (No. 11) and Leydig (No. 10), has estab- 

 lished a fairly satisfactory view of the phylogenetic history of the gill. Mitsukuri, 

 who first made known the structure and nature of the gills of Nucula and Yoldia, now 

 considered to be the most primitive of living lamellibranchs, was the last of these 

 authors mentioned to publish his views -, and he reviewed the work then completed, 

 giving a theory of the phylogenetic development of the gills, which is generally accepted 

 as the true one. He says: "To review the whole matter, the lamellibranch gill was 

 perhaps originally a simple ridge on the side of the body, but, to increase the surface 

 of contact with the water, folds may have arisen on two sides of this ridge. If such 

 was the case, Nucula and Yoldia are still in a stage very little advanced fi'om this 

 primitive condition. In course of time, however, as some of the Lamellibranchiata, 

 either owing to degeneration or some other cause, became incapable of extensive loco- 

 motion, these gills or folds were perhaps prolonged to form tentacular filaments, which, 

 going on in their development, finally produced such complex gill structui-e as we see 

 in Ilytilus, JJnio, Ostrea, and other forms, taking on at the same time functions totally 

 foreign to their original one." 



THE GILL OF YOLDIA. 



The primitive plate gills of these two forms, aheady briefly described, were first 

 studied by Mitsukuri, whose attention was directed chiefly to the former. On acco^mt 

 of poor material, he was not able to examine into the histology of the gill of Yoldia in 

 any detail, though he gave an account of the more important features of its structure. 



As described by Mitsukuri, the plates of the gill are suspended from above by a 

 thick membrane (Fig. 92). Close to its attachment to the gill plates, there is a blood 

 channel running the length of the gill, and a similar vessel is present also in the 

 median line, just above the groove on the ventral side of the gill, which separates the 

 plates of either side fr-om below. The coui'se of the blood in these channels is not 

 known with certainty. Mitsukuri made very little out of the histology of the plates. 

 Contractions. — If a living Yoldia be removed from the shell and examined in sea 

 water, the gills will be seen to possess a deep-red color. The thickened ventral edges 

 of the plates seen fi^om below are light red, but the thinner lateral edges are of a much 

 darker hue. The gills will be observed to be able to contract themselves in a variety 

 of ways and to be very susceptible to stimulation from without. In the first place, 

 they may shorten themselves to a considerable extent from before backward, and, like 

 all the movements of the form, this maybe done very quickly. A contraction may also 

 take place in such a way as to greatly reduce the circumference fr^om side to side and 

 from above downward. This last contraction is a very common one and at times 

 occurs in a curious way. At any point in the gill, three or four plates adjoining one 



