414 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
the latter has a different innervation, while the muscular septum proper is innervated 
by the branchial nerve, and that it is in direct communication with the efferent lacunae 
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 Lovdn (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 staige very little advanced from 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 structure as we see 
in Mytilus, Unio , Ostrea, and other forms, taking on at the same time functions totally 
foreign to their original one.” 
THE GILL OE YOLDIA. 
The primitive plate gills of these two forms, already briefly described, were first 
studied by Mitsukuri, whose attention was directed chiefly to the former. On account 
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 from below. The course 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 from 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 fr om 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 from 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 
