598 THE INVERTEBRATA 



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

 the brachial ganglia a separate nerve, which carries a ganglion on 

 its course, to each arm. 



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. 

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

 brain can be carried out. 



Sepia possesses very large eyes (Fig. 409 C), 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 posi- 

 tion of the lens. When it is relaxed the eye is focussed on the distance : 

 when it contracts, increasing the pressure of the vitreous humour and 

 so pushing the lens forward, the eye is focussed on near objects. 



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. The surface of contact between these cells 

 and the egg is increased by folding. When ripe the ova escape into the 

 genital coelom and pass into the genital duct. This has a terminal 

 glandular enlargement and there are also the nidamental glands, un- 

 connected 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 

 into the funnel and then on to one of the arms (the hectocotylus) 

 which is modified for the purpose of transferring the sperm to the 

 female. In Sepia, the modification shows itself only by the suppres- 

 sion of some rows of suckers at the base of the arm, but in other 

 forms it is profoundly modified. In Octopus, the end of the arm 

 is spoon-shaped and the arm is extended so as to enter the mantle 

 cavity of the female. In other octopods, a cyst, in which the sper- 

 matophores are stored, is formed at the end of the arm ; from it a long 



