34 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



become free, and give rise to tufts of sperms (spermatozoa). 

 Only part of the substance of the mother-cells is used up in 

 this process. Each sperm possesses a small oval head, and long 

 vibratile tail. The walls of the vesicula seminalis, cirrus-sac, 

 and penis contain muscle-fibres variously arranged, and the last 

 is lined by a continuation of the spiny cuticle covering the 

 body. 



The ovarian tubules possess supporting layers, and are lined by 

 germinal epithelium, the cells of which become ova. Each of 

 these has a small vitellus, and large germinal vesicle, with germi- 

 nal spot. In the acini of the yolk-glands a formation of yolk-cells 

 occurs. These contain a large amount of nutritious matter. A 

 yellowish fluid, which can harden into shell substance, is secreted 

 by the shell-glands. 



5. Muscular System. This is made up of bands and sheets of 

 contractile muscle-fibres, by means of which definite movements 

 are effected. When these fibres contract they become shorter 

 and broader, their ends being thus 'brought closer together so 

 that the parts to which they are attached tend to approach. In 

 the case of fibres surrounding a cavity in a circular direction 

 (pharynx, for example), narrowing of the cavity takes place, and 

 the reverse is effected by longitudinal fibres. The movements of 

 locomotion are mainly due to the dermal musculature, a sheet of 

 muscle intimately connected with the skin, and together with it 

 making up the body-wall. The sheet is composed of three layers, 

 external, middle, and internal, the fibres of which take respec- 

 tively transverse (circular), longitudinal, and oblique directions. 

 Closely connected with these are the suckers, which are muscular 

 cups of rather complex structure. There are also a great many 

 muscular bands running through the connective-tissue which fills 

 up the space between the organs. These bands take a dorso- 

 ventral direction, and are also connected together so as to form a 

 close network. 



Locomotion is effected in the following way: The ventral 

 sucker having been fixed, the head-papilla is elongated by the 

 contraction of its circular muscle-layer. The anterior sucker 

 then attaches itself, the ventral sucker being at the same time 

 loosened, and the body is dragged forward by the contraction of 

 its longitudinal muscle-layer. 



6. Nervous System (Fig. 7). Under this term are included 



