}20 



Oue wouM perhaps naturally anticipate that the same order of animalft 

 in which the different genera and species bear, in their adult forms, considerable 

 resemblance to one another, should present a corresponding similarity in their 

 larval state. This, however, is not always the case. We have a striking illus- 

 tration of this in the family of Ephemeridce amongst insects. The genus 

 Ephemera differs from the genus Baetis in the perfect form of the insect very 

 slightly. The one has three caudal setas, the other two. This is the only obvious 

 difference to an ordinary observer. Now look at the larv;e of these two insects. 

 What similarity exists between them ? Amongst the Crustacea, the Afacroura, 

 or long-tailed family— familiar to all in the shape of lobsters and crayfish— bear 

 in their adult form great resemblance to each other; but how wonderfully 

 different are their larval forms. "The young lobster has divided legs like a 

 Mysis ; the Palcemon appears under the form of a 2^oea,, and Peneus under the 

 Nauplius form." The cray-fish undergoes no metamorphosis. Nay, even the 

 larvae of closely-allied genera differ considerably. 



Again, amongst the fresh water Planari, the young ones issue from 

 the eggs as perfect planari, but in a marine Nemertes, a turbellarian worm 

 of great length, the larva appe:irs in the form of a helmet, with a long bristle- 

 like appendage at the top like a plume ; from its shape it was called PiUdium, 

 and was supposed to be a new animal. What are we to gather from these 

 facts, and how are they to be accounted for? Mr. Darwin considers that they 

 demonstrate the absolute independence of old and new structures. He says : — 

 "According to the belief now generally adopted by our best naturalists, aU the 

 members of the same order, or class, the Macrourous Crustaceans, for instance,, 

 are descended from a common progenitor. During their descent they have 

 diverged much in structure, but have retained much in common, and this 

 divergence and retention of character has been effected, though they have passed 

 and still pass through a marvellously different metamorphosis. This fact well 

 illustrates how independent each structure must be from that which precedes 

 and follows it in the course of development." 



I shall conclude this paper with an extract from an article by Dr. Allen- 

 Thomson, who has in a few graphic words described the extreme importance of 

 the study of animal development : — 



"It will be enough at this place to advert to the extensive range of topics 

 which must be embraced in an attempt to trace the history of the first origin and 

 subsequent evolution of all the parts of so complex and various a structure as 

 the body of animals ; and to remind my readers that this department of science 

 professes to describe not merely the successive changes of external form and 

 relation by which the several organs, springing from imperceptible beginnings, 

 arrive at their perfect condition, but also the more minute phenomena of histo- 

 logical development or changes of the several textures, wldch accompany the 

 more obvious formative processes ; that, as in many instances, the complete 

 knowledge of the structure and function of an organ is only to be obtained 



