256 



PRINCIPLES OF ANIMAL BIOLOGY 



through very similar stages, in which the partitions were first straight, 

 then bent, crooked, and finally complicated. 



Another suggestive and perhaps significant individual development 

 is that of the decapod Crustacea (lobsters, prawns, shrimps). The shrimp 

 Penaeus hatches as a nauplius, and goes through several increasingly 

 complex forms (Fig. 211), the last immature one being the so-called my sis 



Badrites 



Anarcesfes 



Pronoritcs 



Ceraiites 



Phylloceras 



Fig. 210. — Biogenetic law illustrated by fossil cephalopods. Edges of partitions of 

 shells start nearly straight and become increasingly crooked, in both evolution of group and 

 development of individual ammonites (like Phylloceras, E). {From Storer, "General 

 Zoology.") 



stage. The appendages through all this development are two-branched. 

 In the adult shrimp, however, the outer one of these branches on the five 

 pairs of trunk appendages is considerably reduced. The lobster has 

 shortened its individual development and hatches as a mysis which has 

 two branches on all appendages; but the five pairs of walking legs have 

 lost the outer branch completely in the adult. The support which these 



Nauplius Protozoea 2oea Mysis , ADULT 



P'iG. 211. — Larval stages and the adult of the shrimp Penaeus, jjeihaps illustrating 

 biogenetic law. Numbcis refer to successive appendages. (From Storer, "General 

 Zoology.") 



decapods give to the biogenetic law lies principally in the fact that there 

 exists a present-day animal called Mysis (from which the larvae of other 

 forms take their name) in which two well-developed branches persist 

 on the trunk appendages of the adult. The lol)ster and shrimp thus 

 pass through a developmental stage which resembles a supposedly more 

 primitive animal. 



In most animals embryonic develoi)ment has undergone many 

 changes, so that steps in development no longer represent accurately 



