LECTURES 'ON EMBRYOLOGY. 



tYieiTirsrses,so that the nurses are born from 

 another kind of Worms, living equally as parasites 

 in those shei's, and which are on that account 

 called grand nurses, so that we have now three 

 generations , Grand nurses observed in the early 

 part of the year giving rise, by a series of devel- 

 opment of their eggs to so called nurses, in which 

 chere are asrain eggs produced which undergo all 

 the changes ef a regular developement, and are now 

 v>orn as Cercaria. And when these Cerearia have 

 2ived as free animals for a certain time, they un 

 dergo the changes which produce Distorfia 



It is a remarkable fact, that the nurses of Cer- 

 -carias bring forth a great many Cercarise, which re- 

 main as parasites; a great many of them being 

 -developed within the body of the shell fish, into a 

 Distoma. WQ have, therefore, three successive 

 generations which differ. The grand nurses give 

 Tise to a generation which resemble them in a cer 

 lain degree, but not in every respect. And the 

 nurses which give rise to*Cerearise ; and by meta- 

 morphosis the Cercariss are transformed into Dis- 

 toma. How the grand nurses are formed, has 

 not been observed directly. But it is known from 

 other species, and it has been observed by Siebold, 

 that the Distoma will mature eggs which will give 

 rise to other ttnimals similar to our grand nurses 

 These which will either grow within the maternal 

 Distoma body, as in this form. (PI. XXXIV, figr. B) 

 where we have here Disroma, {PI. XXXIV, fig, A ) 

 and here, {Fig B) we have its progeny. Butastbis 

 progeny is so different from the parent, there can- 

 not be a doubt but that at a certain period the 

 Distoma lays eggs, and that there is a certain gen- 

 eration which resembles the first starting point of 

 the animal. But whatever may be these changes, 

 there will be always a ^period when the animal 

 will lay eggs. And whatever may be the number 

 of these intervening generations, there will be al- 

 ways a period when the animal will come back to 

 the fundamental type of its species. 



In the Tape-worm, a curious observation has been 

 made by Prof. Eschricht, who has ascertained that 

 the head, when it is furrowed by innumerable 

 joint?, will from time to time cast these joints, and 

 at regular periods reproduce them. The joints 

 present a remarkable uniformity of structure, in 

 each joint there being the various apparatus 

 ovaries and other organs, which are developed in 

 these animals. 



So that each joint is, in certain respects, an indi- 

 vidual by its structure, but remains united with its 

 other joints, forming a series of articulations In 

 such a condition of things, we have certainly an 

 approach to or at least some analogy with what 

 we have observed in the Medusae, which form 

 those piles of individuals called Strobila^ which 

 become free and give tise to as many individuals. 

 En Intestinal Worms such transverse divisions take 

 place ; the animal being free and each ring be- 

 coming as nearly as possible a peculiar individual 

 terming a kind of compound animal, but in a dif- 

 10 



ferent sense from what we have observed among 

 polyp-i, ti'K at a certain period of the year, they 

 castthese rings and scatter about the innumerable 

 eggs which they produce. The quantity of eggs 

 which are produced in each of these animals, and 

 the quantity of eggs which are produced by each 

 individual Warm, is amazing. 



Prof. Owen has computed, that in one single full 

 grown female Ascaris, there were sixty-four mil- 

 lions of eggs developed. Now as it has been ascer- 

 tained by several Entomologists, that Intes- 

 tinal Worms a-nd their eggs have a more persis- 

 tent life than other animals, we should not won- 

 der tbat they have a chance to re-enter the bodies 

 of animals in which they live. It is a remarkable 

 fact, that Intestinal Worms are found generally 

 in particular animals, and that the same spe- 

 cies is not developed in every kind of animal, 

 even if they live under the same circumstances. 

 And now the chance which these various kinds of 

 Tape-worms have of being introduced into ani- 

 mals of the same species as those from which they 

 have been removed, is very great. 



In the Fishes, for instance, the Parasites become 

 a part of the food of the Fishes, and in this way 

 they are transfered into the animals in which they 

 live. Some ef these Intestinal. Worms have ua- 

 dergone the action of boiling water without being 

 killed. Their eggs have been pat under the influ- 

 ence of strong acids without being destroyed. So 

 that we should not wonder, after such experiments 

 have been made, that these animals, having been 

 introduced into the alimentary canals of animals 

 should live to grow and reproduce their species, 

 instead of being digested. 



The eTternal Worms such as live in the water 

 or the earth when they are hatched, present al- 

 ready transverse divisions. They early "assume 

 (Plate XXVII, fig- A | the shape of common artic- 

 ulata. Professors Milne-Edwards, Loven, and KoK 

 liker have traced the changes of several of those 

 Worms. But I see that I have scarcely time to state 

 the leading facts of their history, and I must go on 

 to another subject, 



I shall now endeavor to show that there is a uni- 

 formity of type among the Worms, notwithstand- 

 ing the external differences we observe among 

 them. In these various external Worms (Plates 

 XXVIII and XXIX) we may notice some in which 

 there are no external appendages at all (Plate 

 XXIX, figs. A and B) for instance, theNemertes 

 which is very common on these shores where I 

 have first noticed several species. In Planaria 

 there are also no external appendages (Plate XXX, 

 fig. B). 



In the earth-worm (Plate XXX, fig. A) we hav 

 appendages upon their rings, and although very 

 simple, we have here the first step toward those 

 complicated appendages which we notice in oth- 

 ers. The complications grow out of modifications 

 of those appendages themselves. Instead of stiff 

 hairs scattered about, we may have a brush of 



