HIGH SCHOOL ZOOLOGY. 



229 



and often by teeth on tho valves. In front of the hinge are 

 the umbones, the first-formed parts of the valves ; they gener- 

 ally incline forwards. Three layers may be seen in the shell, 

 the outer brown periostracum, the thick prismatic layer formed 



by the activity of the thickened 

 border of the mantle (Fig. 155), and 

 the nacreous or pearly layer, se- 

 creted by the whole mantle surface. 

 Pearls are the result of repeated 

 layers of this substance being form- 

 ed round foreign particles, which 

 have got between the mantle and 

 the shell. The mantle corresponds 

 in form to the shell, hanging down 

 right and left of the body : in many 

 Lamellibranchs, the margins of its 

 two lobes tend to unite, except in 



Fig. 155.-Dia,^mma«c transec«on f^'^^^' <^° ''^^^^^ ^^'^^^ ^"^ ^^'^ ^^^*^' 

 ofAnodon. (After Ludwig). a^d behind at two points, the 



1, li-aroent; «^, shell ; w, mantle ; . , , n . . j. • ^ 



m\ ita thickened niarsrin ; g\ ,7-, SipnOnS, tO alloW water to get intO 

 outer and it)ner fcill-ulates; /, in the , l /f ±i i.i 'i. i, j. 



niantie-ca\itypoints to the foot, which and out 01 the mantle cavity, but 



contains portiona of the intestine (i) , , . . ■> . • ±-\ 



and reproductive organs; ?i, is in the tlllS UniOn doeS nOt OCCUr in the 



glandular part of the nephridium, tt • J j.t_ • x /tt"!™ lKn\ 



*openingofitsnon-glandularpartinto UniOS, and the Siphons (tig. lob), 



themautie-cavity. ^^.^ ^^^^^ Specialised parts of the 



mantle-margin, which can be fitted together. 



Locomotion is effected by the ploughshare-shaped foot ; mus- 

 cular fibres are hardly developed elsewhere in the body-wall 

 except along the border of the mantle, and especially at its 

 anterior and posterior ends, where the adductor muscles which 

 close the shell are situated. 



Kelated to these muscular masses are the three pairs of nei've- 

 ganglia — cerebral, pedal and parieto-splanchnic — with connect- 

 ing nerve-cords. A pair of flat, triangular, tactile tentacles on 



