MUSEUM OF COMPARATIVE ZOOLOGY. 101 
EXPLANATION OF FIGURES. 
LETTERS. 
The following letters are used to designate respectively : — 
A. == Anterior. mu. == Muscle. 
bac. = Bacillus. mu!. = Muscle, cut eross-wise. 
enc. = Brain. n. op. == Optic nerve. 
jis. tap. = Tapetal fissure. Py ‘= Posterior: 
gl. = Poison gland. pr. = Post-retinal cell-layer of the eye. 
Ing. = ‘‘Lentigen,”=‘‘ Vitreous” grr. = Pre-retinal cell-layer. 
(auct. ). 7. ==> Retina. 
Ins. | = Cuticular lens. tap. == Tapetum. 
Figures 1-24 were all drawn, with the aid of the Oberhauser camera, to the same 
scale (X 515 diam.) from balsam-mounted sections cut from objects stained in alco- 
holic borax carmine (Grenacher’s) and imbedded in paraffine. Figures 1-16 and 
18-24 relate to Agelena nevia; fig. 17 to Theridiwm tepidariorum, C. K. Figures 
1-16, 18, 19, 23, 24 are from preparations by Mr. W. A. Locy; figs. 17, 20-22 from 
preparations by Mr. G. H. Parker. 4 
PLATE I. 
Figs. 1-7. Median faces of successive sagittal sections from the left half of the 
head of a young Agelena nevia, about four days after hatching. The position of the 
portion of the brain nearest to the eyes is indicated at enc. 
Fig. 1. The plane of the nearer surface of the section passes through the middle of 
the anterior median eye, cutting its optic nerve obliquely. The latter emerges from 
the retina immediately beneath the ‘‘lentigen.” The distal ends of the elongated 
nuclei in the ‘‘lentigen ” are scarcely discernible, not being sharply marked off from 
the surrounding substance, nor so deeply stained as at their proximal ends. Behind 
the anterior eye, and beyond its optic nerve, are the muscles which separate the pos- 
terior median eyes, and then pass obliquely forward and downward, in part beneath 
the anterior median eye, in part between it and the anterior lateral eye (compare 
Figs. 2 and 3). Beyond these muscles, and partly obscured by them, is the layer of 
cells composing the median wall of the posterior median eye. The muscle-cells are 
traceable through the “‘ hypodermis ” to the cuticula at the surface of the head. 
Fig. 2. This section embraces a large portion of the lateral wall of the anterior 
median eye, and the middle region of the posterior median eye. In the latter there 
are four well-marked regions, — post-retinal, tapetal, retinal, and pre-retinal. 
Fig. 3. The lateral wall of the posterior median eye is embraced in this section, so 
