No. 2.] THE EMBRYOLOGY OF LIMULUS. 223 



XII. Each of these appendages has upon its posterior surface 

 a large number of rounded quadrilateral lamellae (the number 

 varying with age), the "gill-leaves," and the whole organ is 

 known as the '' gill-book." The first of these appendages to 

 appear is the most anterior one (VIII). It grows out from the 

 body, not as a cylindrical process like appendages I-VI, but as 

 a broad lobe with an oblique insertion upon the ventral surface 

 of the body (Figs. 28, 32 g). At first this lobe consists merely 

 of a fold of ectoderm (Fig. 'j'S) marked off from the ventral 

 surface by an impushing behind, and containing in its interior 

 scattered branching mesoderm cells between which are numer- 

 ous large lacunas. At stage H this appendage shows the first 

 appearance of the gill lamellae in the form of a single fold of 

 the ectoderm of the posterior surface at about midway of the 

 length of the member. Very soon a second and smaller out- 

 growth appears between the first leaf and the body, and so on 

 in regular succession (Fig. 79) the new leaves continuing to be 

 added at the base of the appendage, the outer leaves being the 

 larger and older. 



When the first gill-leaf appears on appendage VIII, appendage 

 IX is budded, and on this the first gill-leaf appears (Fig. 79) 

 when on appendage VIII there are four or five lamellae. The 

 subsequent growth of these appendages and the new gill-leaves 

 is but a repetition of that already given. At the molt with 

 which the telson appears and with which my studies end, 

 appendage X has appeared, but is as yet without lamellae, while 

 appendages VII and IX are well provided in this respect (Fig. 

 80). 



At stage I (Fig. 79) the gills and gill appendages present 

 the following appearances in their finer structure. The ec- 

 toderm has become columnar and has secreted on its free surface 

 a thin cuticula. In the gill-leaves the two walls are united 

 here and there by fine mesodermal filaments which extend 

 between the two walls like the tie rods of an architectural 

 structure. Between these trabeculae are lacunae without definite 

 walls, and in these, scattered blood corpuscles are to be seen, 

 showing that these spaces are in connection with the general 

 circulatory system. These lamellae are shown in cross section 



