ANATOMY AND PHYSIOLOGY. 
Moll. 27 
agrees with the fact observed by geologists that the outer layer of shells 
is better preserved than the nacreous, and this than the fibrillar, so-called 
“ ivory ” layer. 
Muller (275) shows that the shell of the Lamellibranchs is not 
merely a secretory product, but is alive and grows by intussusception. Its 
outermost margin, and the surface turned towards the mantle, always 
remain soft. The calcification takes place by small, round, irregular 
structures, which afterwards become prismatic by mutual pressure ; the 
lamellar structure is a secondary modification. The organic substance of 
the shells originates in the cells, not, as Nathusius asserts, independently 
of them. The fibrillse of the shell follow in their development the direc¬ 
tion of action of the muscles, which are developed along with them. The 
shell is opened by transverse muscular fibres, not simply by the elasticity 
of the ligament ; this action is also assisted by the radial muscles in the 
dorsal aspect of the animal. The transverse muscles of the mantle draw 
the soft margins of the shell against each other, and thus secure its com¬ 
plete closure. Shells are divided into—-(1) Those connected with the 
mantle only at isolated places; most belong to this class. Organic sub * 
stance of nacreous layer membranous. (2) Those connected over the 
whole mantle. Organic substance of nacreous layer, finely reticulate. 
Only in Cyclas. 
According to Krukenberg (219, 220) chitin occurs in Sepiola rondeleti 
only in the jaws, fhe chief part of which it also composes in Octopus , 
Eledone , Sepia, and Loligo. In Spirula peronii it is found chiefly in the 
septa of the shell; in Argonauta the shell is mainly albuminoid. Nautilus 
contains chitin in some quantities, but not distributed as in Spirula. The 
same author also gives an account of the composition of the shell, and 
other skeletal structures (218). 
Cartilage of Sepia contains chitin; Halliburton (158, 159). 
Chromatophores, effects of mechanical stimulation upon, observed by 
Albini (2). 
2 . Muscular System. 
Pawlow (284) has examined the action of the adductor muscle of Ano- 
donta cygncea. Two kinds of nerves are present: motor, producing con¬ 
tractions, and inhibitory, producing relaxation. The motor nerves for 
each muscle spring from the ganglia next it. All the inhibitory fibres 
spring from the two anterior ganglia. 
Varigny (362) finds that the unstriated muscles of most invertebrates 
( Cephalopoda in particular) are :—1, under the influence of the will; 
2, not necessarily “ thermosystaltic ; ‘3, not invariably less irritable than 
striated muscles; 4, sometimes very rapid in contraction and relaxation ; 
5, of variable latent period. This last is shown by the same author (363) 
to be|A sec. in Scaphander Vgnarius , I ~ 5 5 sec. in Sepia officinalis ; it varies 
with the intensity of the exciting current, with the mode of excitation, 
with the weight to be lifted, and with other conditions. 
The adductor muscles of Anodonta retain their contractility for days 
when kept in the blood; numerous facts regarding amount and energy of 
contraction : Bie dermann (19). 
