MEMOIRS OF tHE NATIONAL ACADEMY OF SCIENCES. 32 i 



At this stage the appendages are very thick and hke loose flattened sacs, and the abdomen 

 beyond is htose and baggy. 



My sections were sagittal ones, and, though made on freshly preserved material, were not so 

 satisfactory or desirable regarding histological details as could be desired. 



The "operculum " or first pair of abdominal legs are thicker, though but little longer, than the 

 second pair. Seen in section on the slide, the limb is simply sack-like, with no internal structures 

 or any hairs externally, though the musculature is in part developed. The second pair alone bears 

 the gills; the appendage itself is nearly, if not quite, as long as the first pair, though not so tliick. 

 The muscles are distinctly developed. The legs each bear five leaf-hke sacs (Figs, li, .3, i, 5, a, b, e, 

 d, e), which are from one-half to two-thirds as thick as the leg itself, and gradually diminish in length, 

 the outside one, which is the last to be developed, being minute, the two basal ones being of nearly 

 equal length and from a third to a fourth as long as the entire limb. The gills are hollow, but lined 

 with hypodermic cells, though the nuclei are not visible, this being perhaps due to the mode of 

 preparation or of sectioning. The limits of the cells themselves are not visible, and from the thick 

 protoplasmic, very transparent lining, which does not readily stain with the alcoholic carmine, more 

 or less conical, pointed processes extend out into the cavity, and sometimes meet similar processes 

 from the opposite side, forming slender pillars or trabeculiB. These thin protoplasmic partitions, 

 though suggesting the regular transverse septa of the spider's lung-lamelhe, described and figured 

 by MacLeod * and by Locy, are by no means so distinctly developed and are not morphological 

 equivalents to them. The sliort outer leaves are more or less crumpled, with very thin walls. 



The larval (PI. XX, Figs. 6, Cm, (ib, 6c) were prepared, sectioned, and mounted in the same manner 

 as the embryos, but the histological structure of the gill-sacs is now much more definite and satis- 

 factory to study, the hypodermis consisting of distinctly marked cells, with large deeply stained 

 nuclei. 



The first pair of abdominal legs reach to the end of the abdomen, as already described and 

 figured in my first memoir, and also more elaborately by Dr. Kingsley. They are seen in section 

 to be very thin, being in this respect much as in the adult. 



The second pair of legs (Fig. Oi, 6c, i) is oidy about half as long as the first pair. The hypo- 

 dermis towards and at the base consists of large distinct columnar cells, becoming at the base 

 double. Under the hypodermis, at the base of the appendage, is a double series of rounded, quite 

 large cells; while within the interior is filled with nuclei of irregular size, arranged in short streaks, 

 the cell- walls being obliterated. PI. XX, Figs. 6«, 6c, show five leaves or gill-sacs lined with a 

 hypodermis as distinct as in the appendage itself. The basal or first-formed gill-sac is nearly twice 

 as long as the second one; the succeeding ones diminish in length, the fifth one being a simple fold. 



There are no traces of any transverse septa comparable with those of the lung-leaves of 

 Arachnida. 



The third pair of appendages (PI. XX, Fig. 6c iii) is now simple, not standing out free from 

 the surface, but lying under the second pair. The cells of the hypodermis are large and round, not 

 closely packed and columnar, as in the second pair. As yet there are no rudiments of the gill-sacs. 

 The cells which are to give rise to them are as yet in an inditterent state, and this condition of things, 

 shown in our figure, strongly recalls the structure of the embryonic appendages destined to form 

 the book-lungs of the spider represented by Kishinouye. (His Fig. 34, 1 abd. app., 2 abd. app., etc.) 



This striking similarity in the shape of the deeply stained iniclei and their arrangement j'ust 

 as the book-leaves or sacs are beginning to be formed shows the close similarity of the mode of 

 origin of the appendages in question of the two groups. 



Further observations, both on the embryonic as well as larval structure of Linuilus and of 

 Arachnids, are now needed to finally clear up the points which have been raised by recent in- 

 vestigations. But Kishinouye's statements and his Fig. 34 have, more than any others, convinced 



•MacLeod has stiuliecl the gills of the adult Limiilus by sections, and finds that the inner cavity of a gill-sac is 

 lined entirely with hypodermic cells and has fibrous pillars, besides numerous multicellular glands opening on its 

 upper surface. On the other hand, a single leaf of a spider's lung has within it numerous transverse pillars (quer- 

 pfeiler), each with two nuclei, and a peculiar modification of the plasma, wliich perhaps acts as a muscle to contract 

 the cavity of the leaf. Othor cell elements are wanting. (Arch. Biol, v., 1-34, 1884). 

 S. Mis. 169 iil 



