MKMUIKS OF Till-; NATIONAL ACADEMY OF ailEN( KS. ,'52 i 



At lliis stago tli(> a|>|)ciulu;i:e.s ud; vuiy thick and like loose llatU-iied sacs, and tlie ahdoirien 

 beyond is loose and ba^jf^y. 



My sections were satrittal ones, and, tliiiuj;li made on tVesliIy preserved material, were not so 

 satisfai-tory or desirable re^ardinjj liistolo{;ri<;al details as could be desired. 



The "oijerenliim" oi- lirst pair of abdominal lej;s are thicker, thouf^h butlittle longer, than the 

 second pair. Seen in section on the slide, the limb is simply sack like, with uo internal structures 

 or any hairs externally, thou;ih the musculature is in part ilevelopcd. The second jjair alone bears 

 the gills; the appenda;;;*^ itself is nearly, if not (juite. as long as the lirst pair, though not so thick. 

 The muscles are distinctly develoi»ed. The legs each bear live leaf-like siu;s (Figs. U^.5, 4, ~), a, l/,c, 

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

 the outside oius whitrh is the last to be developed, being minute, the two basal ones being of nearly 

 eipial 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 perliai)s due to the mode of 

 prejiarat ion or i>f sectioning. The limits of the cells themselves are not visible, and from the thick 

 prot<»plasniic, 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 jjroccisses 

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

 thoujjli suggesting the regular transverse septa of the spider's lung-lamellic, described and figured 

 by .MaiLeod 'and by Locy, are by no means so distinctly developed and are not morphological 

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



The larva' ( I'l. XX, Figs. 6, Ga, 6b, Gc) were prepared, sectioned, and mounted in the same manner 

 as the embryos, but the histoh)gical structure of the gill-sacs is now much more detiniteand satis 

 f;ictory to study, the hyiMxlermis consisting of distinctly marked cells, with large deeply staiiu-d 

 nuclei. 



The first pair of abdominal legs rea«h to the eud of the abdomen, as alreatly described and 

 tigured in my first memoir, and also mtue elaborately by Dr. Kiugsley. They are seen in section 

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



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

 dermis towards and at thi^ base consists of large distinct columnar celjs, becoming at the base 

 double. Under the liypoderinis, 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. Ga, Gc, show five leaves or gill-sacs lined with a 

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

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



Tlurre 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 lit) 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 ycrt in an indifferent state, and this condition of things, 

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

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



This striking similarity in the shape of the deeply stained nuclei and their arrangement Just 

 as the book-leaves or sa«s 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 Liniulus and of 

 Arachnids, are now lu-eded 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 ha« ^tnilicd the gills of the .idult LimiiliiH Ijy sections, anil finds that the inner cavity of a gill-sac is 

 liniMl (Mitiridy with liypodormio cells and has fibrous pillars, besides niinicrons multicollnlar glands opening on its 

 np|>er snrfa<•^^ On the other hand, a single loaf of a spider's lung h:is within it nuiuerous transverse pillars (ijuer- 

 pfeiliT), f.i<;li with two nuclei, and a poculi.ar modiliratioa of the plasma, which perhaps acts as a muscle to contract 

 the eavity of the leat. < )thrT eell elements are wanting. ^ Arch. Hiol. v., 1-3-1, 1884). 

 S. iMi.s. J 09 2i 



