THE CEPHALOPODA 329 



along the sides until the most anterior of them reach the mouth 

 (Fig. 291), and becoming united in front of it, eventually surround 

 it completely. 



A paired epipodial outgrowth, the origin of .the funnel, is formed 

 early in development : its two posterior lobes become prominent and 

 bend inwards toward one another (Fig. 290, q\ thus establishing the 

 condition which is permanent in the adult Nautilus (Fig. 276), but 

 finally, in the Dibranchia, they fuse together completely and form a 

 perfect tube. 



All the nervous centres the cerebral, optic, visceral, and pedal 

 are formed separately as proliferations of the ectoderm. The 

 pedal centres give rise, by subdivision, to the ganglia of the arms. 

 The eyes (Fig. 292, A, B) and otocysts originate as invaginations 

 of the ectoderm, which eventually close up. The otocysts arise 



FIG. 290. 



Development of Loligo. (1) view of the cleavage of the egg during the first formation of 

 embryonic cells. (2) lateral view of the egg at a little later stage ; a, limit to which the layer 

 of cleavage-cells has spread over the egg ; b, portion of the egg as yet uncovered by cleavage- 

 cells ; ap, yolk membrane cells ; kp, cleavage-pole where first cells were formed. (3) later 

 stage, the limit a now extended so as to leave but little of the egg-surface (6) unenclosed ; d, 



nbryo is 



eyes ; e, mouth ; u, mantle sac. (4) later stage, anterior surface, the embryo is becoming 

 nipped off from the yolk sac (g). (5) view of an embryo similar to (3) from the cleavage-pole or 

 centro-dorsal area. (6) later stage, posterior surface. (7) section in a median sagittal plane 

 of an embryo of the same age as (4). (8) view of the anterior face of an older embryo. (9) view 

 of the posterior face of an embryo of the same age as (8). Letters in (3) to (9) :-- a, lateral fins ; 



mouth ; ep, outer layer of the embryo ; /i, A /3, /*, /G, the five paired processes (arms) of the 

 foot ; g, rhythmically contractile area of the yolk sac ; h, dotted line showing internal area 

 occupied by yolk ; k, first rudiment of the funnel ; I, sac of the radula ; m, stomach ; IMS, 

 mesoderm ; n, rudiments of the gills ; o, the otocysts ; p, optic ganglion ; q, distal portion of 

 the ridges which form the funnel ; r, vesicle-like rudiment of the intestine formed independ- 

 ently of the parts connected with the mouth ; *, rudiment of the salivary gland ; t, the closed 

 shell sac ; u, the open shell sac, formed by an uprising ring-like growth of the central dorsal 

 area ; w, the mantle-skirt commencing to be raised up around the area of the shell sac. (After 

 Lankester.) 



laterally on the sides of the foot outside the epipodium (Fig. 290, 

 (6), o) ; .they close up at a relatively late period, often retaining a 

 rudiment of the original external canal, and then approach one 

 another till they come in contact in the median line. When the 

 ocular cavity is closed, the external part of the crystalline lens is 

 formed separately from the internal segment. At the sides of 

 the optic ganglia a pair of cellular masses, formed by ectodermic 

 invaginations, becomes the white bodies of the adult (Fig. 290, c) ; 

 they are the relics of a pair of embryonic ganglia (lateral cerebral 

 lobes). 



The coelomic cavity is hollowed out in the mesoderm as two 

 symmetrical spaces, right and left of the intestine ; it gives rise to 

 the kidneys and the pericardium. The two kidneys are formed 

 independently of one another in their definitive positions. The 

 heart is also formed from the pericardial wall as two paired 

 rudiments. Finally, a portion of the coelomic wall gives rise to 

 the gonad. 



