80 



differ in staining property, size of fibres, course in the brain and the skull, and 

 in origin. 



The statocyst arises from an invagination of the ectoderm which forms 

 laterally on the posterior side of the embryo. The bottom of the invagination 

 becomes large and rounded and projects inward until it reaches the. middle of 

 the posterior part of the head and is separated from its mate by a thin septum 

 only. The cavity of the statocyst remains for some time in communication with, 

 the surface by a long, narrow duct which is attached like the neck of a retort, 

 to the side of the cavity and leads to the posterior surface of the embryo. In 

 the adult the outer opening seems to be closed but the canal remains as a 

 slender ciliated duct which is named from its discoverer, KoeUiker's duct. It 

 leads thru the skull from its inner opening at the front end of the statocystic 

 cavity just above the crista, downward and slightly backward to the lower sur- 

 face of the skull. Here the duct seems to end but it is possible that it leads to 

 the surface. 



The Eye. (Plate III figs. 20 — 22). The eye of the higher Cephalopoda is very 

 similar to the vertebrate eye ; in fact the two structures furnish a most remarkable- 

 case of convergent development, for while differing essentially both in the origin 

 of their parts and in structure , the eyes are so similar that a large part of the 

 terminology of the vertebrate eye is applicable to the Cephalopod eye. In the 

 young embryo of the squid an oval area of ectoderm at each side of the embryo 

 becomes differentiated from the surrounding flat or cubical epitheUum and forms 

 the aniage of the retina. The cells of the anlage become greatly elongated 

 and the nuclei take up different positions at the middle or at each end of the 

 cells so that, although remaining one layered the epithelium appears to be several 

 layered. The cubical epithelium at the margin of each retinal plate forms a 

 circular ridge which gradually becomes a fold surrounding a pit and finally the 

 outer wall of a vesicle , the inner chamber of the eye. From the walls of the 

 optic vesicle . the retina , the lens , and the inner portion of the ciliary body are 

 formed. The outer wall of the vesicle consists of two sheets of ectoderm separ- 

 ated by a small amount of mesodermal tissue. The mesodermal tissue forms the 

 sclerotic coat and the suspensory ligament of the lens, and the ectoderm produces 

 the lens which consists of two segments, one formed in the inner chamber by 

 the inner layer of the ectoderm and one formed in the outer chamber by the 

 external sheet of ectoderm. Two circular folds also containing both ectoderm and 

 mesoderm arise around the optic vesicle. The inner fold grows over the optic 

 vesicle and forms the iris. The outer fold also grows over the vesicle and enclosing 

 the outer chamber forms the cornea and the capsule of the eye. The outer 



