522 THE INVERTEBRATA 



mantle, while in the brachial ganglia there are separate centres for 

 gripping by the suckers and for letting go. 



From the cerebral ganglia there run forward a pair of nerves which 

 end at the border of the buccal mass in a pair of superior buccal ganglia ; 

 a circumoesophageal commissure links up these with the inferior 

 buccal. From the visceral ganglia there is a pair of nerves running to 

 the very prominent stellate ganglia in the mantle; there is also a 

 visceral loop which sends off branches to the gills and at its posterior 

 limit bears the gastric ganglion between the stomach and the caecum. 

 The infundibular ganglion gives off a pair of nerves to the funnel and 

 the brachial ganglia a separate nerve to each arm which carries a 

 ganglion on its course. 



In the dissection of the nervous system a general view of the 

 different parts of the brain is best obtained by making a longitudinal 

 vertical section with a sharp scalpel. Such a section is shown in Fig. 

 381. Afterwards the dissection of the nerves coming away from the 

 brain can be carried out. 



Sepia possesses very large eyes, similar in their structure and 

 development to those of a vertebrate. In the embryo, the eye 

 originates as an ectodermal pit, the lining of which forms the retina 

 and the contents of which become the vitreous humour. The pit 

 closes up and at the point of closure the interior part of the lens is 

 formed. Later appears a circular fold which forms the iris, limiting 

 the pupil of the eye and forming an outer eye chamber which is finally 

 enclosed by the growth of a cornea. The external half of the lens is 

 formed at the same time. A special ciliary muscle regulates the 

 position of the lens, increasing the pressure of the vitreous humour 

 and so pushing the lens forward (Fig. 384 C). 



The ovaries and the testes are simply parts of the wall of the coelom. 

 The ova are cells of large size ; they are nourished by other peritoneal 

 cells, the follicle cells, which surround the ova and pass on food from 

 the special blood supply. Their surface of contact with the egg is in- 

 creased by folding. When ripe the ova escape into the genital coelom 

 and pass into the genital duct. This has a terminal glandular enlarge- 

 ment and there are also the nidamental glands, unconnected with the 

 genital ducts, which have already been mentioned. These secrete an 

 elastic substance which forms the egg envelope. 



The sperm pass similarly into the genital coelom and then by a very 

 small aperture into the sperm duct which is modified to form in turn 

 the seminal vesicle, the prostate gland and the terminal reservoir, 

 called Needham's sac. All these play their part in the formation of 

 the remarkable spermatophores, elastic tubes which by an elaborate 

 arrangement burst and liberate the spermatozoa after copulation. The 

 spermatophores are passed directly from the extended genital papilla 



