638 i.ECTURE XXIV. 



subdivided and disappears, whilst the intestinal canal is completed. 

 The last remnant of the internal yolk-sac appears to be indicated in 

 the Octopus by the anterior caecal prolongation of the crop. At the 

 later periods of development the respiratory movements are vigorously 

 performed by the alternate dilatation and contraction of the mantle, 

 and by a corresponding erection and depression of the funnel : the 

 ink-bag (e) is now conspicuous by the colour of its contents, which 

 are sufficient to blacken a considerable quantity of water, and the 

 little Cephalopod is thus provided with the means of concealing itself 

 from any enemy that might be prepared to devour it upon its emerg- 

 ence from the defensive covering of the ovum. At the period of ex- 

 clusion five of the layers of the dorsal shell (f) of the young cuttle- 

 fish have been formed ; but, except the nucleus, which is calcified, 

 they are horny and transparent. The lateral fins are not merely 

 developed, but are broader than in the mature animal ; and the ce- 

 phalic arms are furnished, not only with their suckers, but with a 

 basal web ; so that the young Sepia is enabled to swim either for- 

 wards or backwards, and its eyes have acquired the requisite deve- 

 lopment to warn it of an approaching enemy, or direct its course to 

 its appropriate food. 



Thus in the class at the summit of the Molluscous, as in that which 

 crowned the Articulate, series, there is no metamorphosis. The ce- 

 phalopodic character is manifested long before the parts of the em- 

 bryo are completed : even the dibranchiate peculiarity of the infun- 

 dibular cartilages is recognisable when, as yet, only the visceral sac, 

 the funnel, the eyes, and the gills are sketched out. No phase or 

 form of Invertebrate existence below the Cephalopod is even transi- 

 torily manifested. Before the ciliary action is visible on the germ- 

 mass, the parts that exhibit it bear unequivocally the cephalopodous 

 stamp. Whilst the funnel remains ununited along the ventral aspect, 

 whilst the arms are without suckers, and the prominent eyes unpro- 

 vided with orbits, the characters of the lower order of the Cephalo- 

 poda are indeed temporarily exhibited, but the gills at no period show 

 the tetnibranchiate condition. 



Were growth superinduced at any arrested stage of Cephalopodic 

 development, no known inferior form of MoUusk would result. No 

 arrested stage of development of any higher animal would produce 

 any thing like a Cephalopod. 



There is so small a part of the multifarious and diversified details 

 of the anatomy and physiology of the Invertebrata from which the 

 practical Surgeon can directly profit, they seem so remotely connected 

 with human physiology, the animals themselves are so diflferent in 



