PROF. A6ASSIZ 7 S 



(PLATE VITI SCORPION.^ 



Articulata that of Insects. And before begin- 

 ning this investigation, I will simply mention that 

 the group of Articulata, as it is now circumscribed, 

 has not always been considered as containing only 

 three classes. A great number of divisions and 

 other arrangements were, at various periods, at- 

 tempted by Naturalists. The &piuers, for instance, 

 were considered as one entirely distinct class, 

 placed between Crustacea aad Inseers, though I 

 am of opinion that they are better united with the 

 Insects, owing to their structure, as well as their 

 natural development. 



Among Articulata, groups have been introduced, 

 which were formerly placed ia other great divi- 

 sions. For instance, the Barnacles were long con- 

 sidered as Shells, from their external coverings, 

 which are really shells j but their anatomical struc- 

 ture has proved a relation between them and Artic- 

 ulate animals, and really a close relation to Crusta- 

 cea proper so close a relation to Crabs and Lob- 

 sters, that,et the present time, no Anatomist doubts 

 that the Barnacles must be placed in one and the 

 same class with them ; though perhaps among 

 Zoologists, there may be some who still think that 

 the external form should be taken into considera- 

 tion, and not overruled by the internal structure; 

 but such doubta deserve scarcely any longer no- 

 lice. 



As I mentioned in the last lecture, intestinal 

 worms were placed among Radiata, but they are 

 proved to be Artieulata, since the nervoas system 



has been lately discovered by Bfr. Blancnard m a] "I 

 the principal types of intestinal worms, and foand 

 to agree, but with some modifications in its general 

 arrangement, with that of Articulata. The Infuso- 

 ria were also formerly arranged among the Radi- 

 atu, bat now their stracture is more extensively 

 known, they should be scattered and arranged 1 

 among various classes, according to their inner 

 organisation and mode of growth some belong- 

 ing to the Worms, and being only the young, or 

 embryonic condition of worms of Planaria, for in- 

 stance ; others belonging to the vegetable kingdom, 

 and being also embryonic conditions of various 

 Alggcr; and others still, belonging to the Crustacea, 

 as for instance the Sotifera. It is remarkable time 

 the extensive investigations made upon the Infu- 

 soria, the object of which was to illustrate the uni- 

 form structure of these animals as a class, go to 

 show that the class ought to be broken up as a na- 

 tural group, and distributed among various other 

 classes. 



How much remains to be done among tke small 

 organized beings,whieh hare to be investigated by 

 the microscope, will be at once understood when I 

 mention that, for instance, the egg of the Mosquito- 

 like animals whose embryonic changes are repre- 

 sented in Piate VII, figs. A, and E., was first consid- 

 ered as an Alga.and described as a species of Gloc- 

 onema, before it was found to be a Musquito-like 

 insect. 



[PLATE VII. EGGS OF 



The great class of Insects is particularly remark- 

 able for the metamorphoses which these animals 

 undergo. And you may at once perceive how dif- 

 ficult it must be to trace all the changes of these 

 animals when I mention, that the perfect being- 

 the perfect insect may be an aerial animal, provided 

 with wings, and flying about; when in another con- 

 dition, it is Quietly buried in the soil, immovable, 

 not taking any food : or, in another condition, it is 

 an aq-uatic worm, swimming freely in Ihe water. 



Under such circumstances, unless there ia an op- 

 portunity to trace all these successive changes, you 

 see how mistakes, as gross as the one to which I 

 have alluded, may be made. Naturalists are now 

 aware of the possibility of such mistakes, and do- 

 not consider an investigation as perfect, as long a 

 the direct connection between the facts in any giv- 

 en case has not been ascertained by continuous* 

 observations. Articulata undergoing such exten- 

 sive changes, must, therefore, be studied in many 



