ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
487 
The egg contains much yolk, and segmentation is markedly unequal. 
Gastrulation is by epibole, a vitelline mass which constitutes the future 
endoderm and mesoderm being enclosed by the overgrowth of the ecto- 
derm. The two primitive mesoderm cells appear late, after the closure 
of the blastopore. Both mouth and anus are derived from the blastopore. 
The free-swimming stage is very short, and by the fifteenth day the 
tube appears as a simple ring secreted by the collar. As contrasted 
with that of Serpula , the embryonic life is condensed. 
Eyes of Polychaetes.* — Dr. Bichard Hesse has investigated the 
structure of the so-called eyes in a great number of Polychsetes, in- 
cluding both predatory and sedentary forms. In the littoral predatory 
forms, the conditions found in the species of Nereis are typical. The 
eyes are little vesicles, with cellular walls formed by an invagination of 
the epidermis. In some cases (as in Nereis) the vesicles are completely 
closed, their outer surface being covered by the cuticle. In other cases 
(Phyllodoce, Eunice , &e.) the vesicle retains an opening to the exterior, 
and through this the cuticle is connected with the central refracting mass. 
The wall of the optic pit is formed first by a double layer of unpig- 
mented cells (outer and inner cornea), then by a pigmented layer 
(retiua). The pigmented layer is composed of two kinds of cell : — 
(1) sensory cells, which are connected proximally to nerve-fibrils, and 
distally are continued into rod-like structures ; (2) secretory cells, which 
bear fibrils continuous with the refracting substance. The secretory 
cells secrete this refracting substance, which is analogous to the cuticle 
formed by the other epidermal cells. In Pliyllodoce the refracting sub- 
stance takes on more of the characters of a true lens. The segmental 
eyes of Lysidice viridis (palolo) have a totally different structure. They 
consist of a bundle of greatly elongated cylindrical cells imbedded in a 
mass of pigment, and proximally continuous with fibrils which pass into 
the ventral nerve-cord. The author believes that they are organs for 
the reception of light, though there is no reason to believe that they can 
form images. 
The author has also made out some fresh points in connection with 
the eye of Alciope. In the retina he finds sensory cells and secretory 
cells, as in Nereis. The sensory cells are continued distally into rods, 
and the centre of the rod contains a slender fibril, apparently of nervous 
nature, which bears a little knob at its distal extremity. Proximally 
the fibril seems to be continued into the sensory cells itself. The author 
believes that the remarkable knob is the end-organ of the sensory cell. 
Further, he believes on morphological grounds that the eye of Alciope 
has some power of accommodation. 
The sedentary Polycheetes show, in regard to their eyes, much 
greater variety of form than the predatory forms. Their eyes are of 
two main types, (1) cup-shaped, (2) epithelial. The epithelial eyes are 
merely patches of specialised cells continuous with the epidermis. They 
may consist of single cells with a mantle of pigment, or of a number of 
such cells. There are two main types ; in the one, the sensory cells 
are continued into receptive rods ; in the other, the sensory cells secrete 
a lens-like substance which is continuous with the cuticle. In the cup- 
* Zeitschr. f. wiss. Zool., lxv. (1899) pp. 446-516 (5 pis.). 
