THE MOLLUSCA 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 invagination 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 seerete the shell. 
The branchiae or ctenidia 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 
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- 
2) 
Fig. 18. 
Fie. 17. Veliger of Littorina, ventral view, x 80. 
Trochosphere of Myzomenia banyulensis. e, eye; f, foot; m, mouth; ma, mantle; 
A, after 36 hours; B, after 100 hours. jl, pa.e, pallial cavity ; 7.7, right liver lobe; s, 
flagellum ; v, velum. (After Pruvot.) stomach ; ¢, 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, ete., 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. 119, D) ; in Gastropoda: in the 
Aspidobranchs, Paludina, bithynia, Calyptraea, Crepidula, Nassa, the 
Heteropoda, and the Pulmonata. The otocysts only are formed 
