LECTURES ON EMBRYOLOGY. 



93 



&s a very thin membrane within the external 

 Boating (Fig. K) of the animal. 



Next the yolk mass within the animal gives rise 

 to an alimentary canal, and at that time the aci- 

 aial is hatched (fig. M). Before it is hatched, it re- 

 sembles by no means the perfect animal. It has a 

 shell. This is a most remarkable fact, It has a 

 shell, though when fully grown it will resemble the 

 Hollusca, without any shell, This shell is entirely 

 !ost before the form of the perfect animal is 

 assumed, (Plate XLII, fig. M). These prgans 

 around the mouth are not yet distinct. Early in 

 life, however, a hearing apparatus exists a kind 

 of sac, resembling the lowest form of ears, which 

 disappears almost entirely in the perfect animal. 

 And the eyes are not yet seen. Kow these changes 

 are brought about, has not yet been established, as 

 the intermediate steps from this condition to the 

 perfect animal have not yet been traced. Indeed, 

 there is a great difficulty in all embryonic investi- 

 gations, in tracing the further growth of the germ. 

 After they have been hatched, they die generally 

 in confinement. It is much easier to trace it at 

 first, than te trace it when it is undergoing its me- 

 tamorphosis to assume the final form of the ma- 

 ture animal. And in this there is more left to in- 

 vestigate than in any other department of Zoolo- 

 gy. It is even to be expected that many animals 

 described as perfect, will be found to be only the 

 young state of other well known animals in their 

 fall grown condition. I cannot, for instance, help 

 thinking that the new genus established by Prof. 

 Muller, under the name of Astinotrocha (Plate L, 

 fig, F), is only a yoang Gasteropod of the family 

 of Doris. 



However, we can learn one great result from this 

 fact, here that the shell in Gasteropoda is not a 

 character of superiority ; that those animals which 

 have a shell, so far from being of a superior type, 

 ought to be considered as the lower ones, as there 

 are many which have shells in their embryonic 

 condition, and afterwards cast them. It has been 

 ascertained by Prof. Loven that all the naked Mol- 

 iusks have a shell when young, and that they all 

 cast this shell as soon as they leave their embry- 

 onic envelope. But though I would now consider 

 the Gasteropoda which have shells as uniformly 

 inferior to those which are naked, this conclusion 

 will probably not be admitted without controversy 

 r oy Zoologists, 



But when we consider the peculiar forms which 

 existed among the shells of former geological 

 ages, especially in the oldest periods, we may sat- 

 isfy ourselves that this conclusion is correct. 



In the first place, let me observe that these em- 

 bryonic shells have a simple margin (Plate Lll.fig. 

 K). Their opening is entire as are the shells of 

 many other Gasteropoda, the Helix, the Trochus, 

 the Turbo, the Natica, &c. But there are many in 

 which the margin of the shell is prolonged into a 

 fetbe, a respiratory canal, as the Fustis; Murex, 



&c., through which the respiratory tube can be 

 protruded and retracted. 



Now if we are allowed to consider the order of 

 succession of fossil animals as of any value, we 

 would have a hint to appreciate the value of these 

 shells. On a former occasion I proceeded precise- 

 ly in a reverse way from the investigation of the 

 types as they are well known. From their anato- 

 my in the present epoch, I proceeded to show that 

 the order of their appearance in geological time 

 agrees with the gradations as they are formed in 

 oar days ; and concluded that the more ancient 

 were the lowest, because they resembled most the 

 lowest of our days. But having once ascertained 

 such facts very extensively, we are prepared to 

 compare the most ancient shells with our shells, to 

 ascertain which in the present creation should be 

 considered as lowest, and we find that the more 

 ancient univalves or Gasteropoda have a simple 

 shell. And that those with a notch are of more 

 recent date. And this is so constant that Paleon- 

 tologists have not yet found one shell with a respi- 

 ratory tube among those of the oldest deposits. 

 Then we should conclude that those which have 

 an entire opening are lower ^ and those which 

 have a notch are higher; and as those ancient 

 shells resemble the type of the embryonic shells of 

 the present age, we should further conclude that 

 those which have a shell at all are lower than 

 those which have none} and that our naked Gas- 

 teropoda should be considered as the highest in 

 the group. But there is one point in the structure 

 of Mollusca which is worth our attention, and 

 wliich throws light upon embryonic phenomena ia 

 general, to which I will allude before concluding. 



The circulation of the blood in Mollusks does 

 not take place as it is observed in other animals. 

 We have here {Plate LHI, fig. B) no continuous 

 blood vessels passing from the heart into arteries, 

 and then dividing gradually into some branches to 

 unite again into complicated tubes to open into 

 the heart again to form veins indeed we have 

 not a closed circulation. It is but a few years 

 since it was known that the blood can be circulated 

 through the body without being moved by a closed 

 system of vessels. The impression has universally 

 been that the circulation is regulated from a cen- 

 tral organ propelling the blood which is circulated 

 through vessels which go on branching into small- 

 er and smaller tubes, and then the blood is col- 

 lected again and brought back again into a central 

 cavity ; so that the circulation would imply a reg- 

 ular circle of this movement of the blood. 



Now in Mollusca, in Haliotis for instance, (to de- 

 scribe only one case) we have blood which is mov- 

 ed by the heart into a tube (Plate LHI, fig. B) 

 which is not gradually branching, and which sends 

 out only a few vessels and then enlarges into a 

 wide cavity. Indeed the vessel is lost, and the 

 blood is emptied into a large cavity in the anterior 

 part of the body. And from this cavity, arise va- 

 rious little vessels, which circulate through all th 



