LECTURES ON EMBRYOLOGY, 



are very few which differ from the Gasteropo 

 da by having En-like appendages on both sides of 

 She body. Then the bivalve shells, called Lamelli- 

 branchia, and the Terebratula, under the name of 

 Brachiopoda, and also the Barnacles orCirripe- 

 dia. That the Barnacles belong to the great type 

 of Articulata I have already shown. The Brachi- 

 opoda ought to be combined with LameUibranchia, 

 having the same structure and differing only by 

 secondary modifications, and the Pteropoda united 

 with Gasteropoda, being merely an embryonic type 

 of that class. In that manner three classes only 

 remain : the Acephala or clam-like shells, the Gas- 

 teropoda or snail-iike, and the Cephalopoda or cut- 

 tle fish-like, 



The order of gradation of these three classes, 

 according to their organisation, is very easily es- 

 tablished. Among Acephala the Bryosoa are the 

 lowest, next the Ascidia, and next the bivalves:; 

 the Brachiopoda being irregular, lower than the 

 regular ones, as the Clams. 



[PLATE LIY COMPOUND AND SIIKJLE BRTO- 



ZOA | 



Among Gasteropoda, the Pteropoda would be the 

 lowest, being, as we have stated before, a mere 

 embryonic type, reminding us of the germs of the 

 snail-like Mollusca. which, when full grown, have 

 a flat foot upon which they walk, among which 

 we would place lower those which have a shell du- 

 ring all their life ; nest, and above all, the naked 

 Mollusca, which cast their shells at an early period 

 of their life. It is a very curious fact that the na- 

 ked Mollusca are born with a shell which they often 

 cast afterwards, showing that the shell is a char- 

 acter of real inferiority. And even among the 



with, a shell,those are inferi- 



or which have the aperture entire. The order of 

 succession in time shows that those shelly Gastero- 

 poda with an entire aperture appeared before those 

 which have either a long tube, or a mere notch for 

 introducing the water to the respiratory organs^ 



The class of Cephalopoda will stand highest, as 

 has always been admitted by naturalists. But we 

 shall consider as the lowest type, those which are 

 provided with a shell. That the chambered shells 

 were innumerable in the former geological periods, 

 especially in the older and middle ages, is very 

 well known, whilst the naked ones occur most 

 abundantly in our days; and only two genera oc- 

 cur in the present creation, with, a shell divided 

 into two chambers, united by a siphon. 



The order of succession in time is a guide quite 

 as safe to appreciate the gradation of types as 

 organization itself 4 so that where the information 

 upon the one point is deficient, we can refer to the 

 other. In more than one instance we have seen it 

 coincide in so striking a manner with the series 

 which the intimate structure had revealed to us, 

 that we can no longer resist such a conclusion. 

 We could say with confidence, that the order of 

 succession corroborated the inferences derived 

 from organic gradation, and, vice versa, that or- 

 ganic gradation illustrates the order of succession 

 in time. 



For the class of Acephala we have been able to 

 establish a natural series, upon evidence derived 

 from internal organisation. Succession in time 

 gives us the same series ; that is to say, the Bryo- 

 zoa are highly numerous in the oldest fossil- 

 iferous strata; then among the Bivalves, the Bra- 

 chiopoda, which have a shell with two unequal 

 sides, and the anterior and posterior extremities 

 symmetrical, follow next in innumerable variety; 

 the groap of Oysters comes after the Brachiopoda, 

 and finally the regular Bivalves, with equal sides 

 and unequal extremities, tending towards a well 

 marked bilateral symmetry, with diversified ends 

 of the body, come last. 



We see here the order of succession in time 

 agrees fully with the order given by organic gra- 

 dation. 



It remains now only for me in my next lecture, 

 to present a rapid and condensed sketch of the 

 embryology of vertebrated animals, in order to 

 show that there we have a uniform type even 

 among the highest living beings to conclude this 

 course of lectures. 



