FROM EMBRYOLOGICAL DATA. 23 



lations. In the aquatic species of that kind, there are external, gill-like, respiratory 

 organs, while in the terrestrial ones the respiration takes place through stigmata and 

 tracheae. In others the head is more developed, the joints of the body less uniform, 

 appendages to some joints are developed, and the whole larva assumes more fully 

 the appearance of a terrestrial animal. In a higher condition still, we see these 

 larvae brightly colored, the head well distinguished, peculiar feet attached to the 

 anterior rings of the body, and feet of another kind to several posterior joints ; larvae 

 of this description begin to resemble the perfect insects which are developed from 

 them, in almost every respect save that they are destitute of wings. In numbers of 

 them there are, in addition to the legs, appendages developed upon all the rings of 

 which the body consists, which give them a most remarkable appearance. Now, if 

 we take into view all these different forms of larvae throughout the class of Insects, 

 without reference to the peculiar types of winged insects which arise from them, 

 and institute a comparison between them and Worms, we cannot fail to be struck 

 with the remarkable analogy which exists between them. The naked larvae corre- 

 spond to the naked Worms, and the larvae provided with appendages resemble the 

 Dorsibranchiate and Tubulibranchiate Worms in so close a manner, that particular 

 genera of Worms might be singled out as corresponding most closely to different 

 forms of larvae among Insects. The larvae of Limacodes, for instance, can be con- 

 sidered as terrestrial representatives of the genus Polynoe ; the larvae of Bombyces 

 correspond to the Nereid Worms, while some among the larvae of Papilio proper, 

 with their protractile branching appendages upon the neck, remind us of Terebella. 

 It would seem as if the larvae of Lepidoptera began their life in a condition analo- 

 gous to that of the higher Worms, and as if they had no analogy to the lowest types 

 of Worms, that is, to the Helminths or Intestinal Worms. However, this were a 

 very incomplete view of the subject, for these larvae within the egg show a still 

 lower organization, and forms and structures which are as closely analogous to the 

 forms and to the structures of Intestinal Worms as those of full-grown larvae are to 

 the higher Worms. 



Now the special characters of Worms as a class consist precisely in the peculiar- 

 ities which distinguish the caterpillar from the perfect insect. Their body consists 

 of numerous uniform joints, the head being scarcely distinguished from the body, or, 

 at least, the head only being distinguished, the appendages to the joints being 

 diversified equally upon all the joints, or entirely wanting, and wings never being 

 developed upon them in any of their stages of growth. Further analogies might 

 be traced, there being W T orms with external gills, as we notice among certain larvae 

 of insects ; others, the Earthworm for instance, with aerial respiratory organs, as are 

 most larvae of Insects. Some are entirely blind, others provided with many ocelli. 

 The internal structure also might be compared, with the same results ; but it will 

 suffice to hint at it, to recall at once to the minds of zoologists and anatomists the 

 difference which exists between the internal structure of larvae and that of the per- 

 fect insect, and the resemblance the larvae show, on the contrary, to the structure of 

 Worms. 



The question respecting the position of Crustacea among articulated animals is 



