THE MOLLUSC A 



29 



spreads from its point of origin in the form of a pallial thickening, 

 which may be only slightly concave, or it may be deeply invagin- 

 ated and afterwards evaginated. The imagination is caused by 

 the very rapid proliferation of the epithelial tissue in the neighbour- 

 hood of the pallial ridge. When evaginated these epithelial cells, 

 having again become external, begin to secrete the shell. 



The branchiae or denidia originate on the inner surface of the 

 mantle as tegumentary projections in the form of papillae or of 

 filaments arranged in series (Fig. 229). 



The Nervous System and Organs of Sensation. The various pairs 

 of nerve-centres arise separately, and usually as thickenings of the 

 ectoderm at the points where ^ 



they are formed. In certain N. // 



cases, however, the nerve-centres 

 are developed by the more 

 primitive process of invagina- 

 tion, as may be seen in the case 

 of the cerebral ganglia of Den- 



FIG. 17. 



Trochosphere of Myzojnenia banyulensis. 

 A, after 36 hours ; B, after 100 hours, fl, 

 flagellum ; ?, velum. (After Pruvot.) 



r.l 



FIG. 18. 



Veliger of Littorina, ventral view, x 80. 

 e, eye ; /, foot ; m, mouth ; ma, mantle ; 

 pa.e, pallial cavity ; r.l, right liver lobe ; s, 

 stomach ; t, tentacle ; v, velum. 



talium, Vermetus, Cavolinia (paired invaginations), Yoldia (Fig. 

 16, c.g), and Dreissensia (an unpaired invagination), a portion of 

 the cerebral centres in Pulmonates, and the cerebral, pedal, and 

 visceral ganglia of the Unionidae. When the cerebral ganglia are 

 formed by invagination, a single pit, or a pair of pits, is formed 

 on the oral side of the apical plate, from the bottom of which 

 ganglion cells are budded off; the remainder of the invagination 

 goes to form the labial palps, etc., of Lamellibranchs, and probably 

 the rhinophores of Gastropods. 



The eyes also, including the pallial eyes of the Pectinidae, and 

 the otocysts are sometimes developed from ectodermic thickenings ; 

 but in many cases these organs are formed by invagination, for ex- 

 ample, in various Cephalopoda (Fig. 1 1 9, D) ; in Gastropoda : in the 

 Aspidobranchs, Paludina, Bithynia, Calyptraea, Crepitlula, Nassa, the 

 Heteropoda, and the Pulmonata. The otocj'sts only are formed 



