24 THE CRUSTACEA 



order from before backwards. The limb -buds early become 

 bilobed and grow out into typically biramous appendages which 

 gradually assume the characters fcjund in the adult. With the 

 elongation of the body, the dorsal shield of the nauplius begins 

 to project posteriorly as a shell-fold, which becomes the carapace. 

 The rudiments of the paired eyes appear under the integument of 

 the head, but only become pedunculated at a comparatively late 

 stage. 



The course of development outlined above, leading from the 

 nauplius to the adult form by the successive addition of somites 

 and appendages in regular order, agrees so well with tlie process 

 observed in the development of the typical Annelida, that it must 

 ])e regarded as the most primitive. It is most closely followed by 

 such Branchiopoda as A])us. and Branchipus, and by some Copepoda. 

 In the majority of Crustacea, however, this primitive scheme is 

 more or less modified. The earlier stages may be passed through 

 within the egg or in the maternal l)rood-chamber, so that the larva 

 only begins to lead an independent existence at a stage more 

 advanced than the nauplius. Further, the gradual appearance of 

 the successive somites and appendages may be accelerated, so that 

 great advances in structure take place at a single moult. For 

 example, in the Cirripedia, the latest nauplius stage gives rise 

 directly to the so-called 6'y/j5ns- larva, which possesses all the 

 appendages of the adult. Another common modification of the 

 primitive method of development consists in the accelerated appear- 

 ance of certain somites or appendages, disturbing the regular order 

 of development. This modification is especially found in the 

 Malacostraca, in which, even among those which have most closely 

 adhered to the primitive order of development, the last pair of 

 abdominal appendages usually make their appearance before those 

 immediately in front of them. The same process, carried further, 

 leads to the very jDeculiar larva known as the zoea, in the tyj)ical form 

 of which, found in the Brachyura, the posterior five or six thoracic 

 somites are greatly retarded in development, and are still repre- 

 sented by a short unsegmented region of the body at a stage when 

 the abdominal somites are fully formed and even carry appendages. 



A remarkable phenomenon observed in the development of a 

 few Malacostraca is the temporary retrogression and even dis- 

 appearance of certain appendages which redevelop in later stages. 

 The best- known instances of this will be further alluded to in 

 describing the development of the Penaeidea among the Decapoda. 



In addition to the nauplius and zoea there are many other types 

 of Crustacean larvae distinguished by special names, though, as their 

 occurrence is restricted Avithin the limits of the smaller systematic 

 divisions, they are of less general interest. We need only mention 

 here the metanaupliics, a vaguely defined stage following the nauplius, 



