210 INVEETEBEATA 



CHAP. 



Three pairs of maxillipedes are, however, developed, and on them 

 consequently devolves the whole function of locomotion. The hinder 

 segments of the thorax are completely suppressed, and no trace of 

 appendages is found on the abdomen (Fig. 157). 



The Zoaea of the Anonmra is very similar in general appearance to 

 the Zoaea of Caridea, but it only possesses two pairs of maxillipedes, 

 and, generally speaking, no trace of the hinder thoracic appendages 

 is present at birth although they appear after the first moult, and 

 the rostral spine is always very long and sometimes enormously 

 elongated. Finally, the Zoaea of the Brachyura, while agreeing in 

 most points with the Zoaea of the Auomura, differs from it and all 

 other Zoaeas in possessing a long mid-dorsal spine sloping backwards. 



The very same controversy which developed concerning the 

 meaning of the Nauplius, raged over the significance of the Zoaea. 

 Some, like Dohin (1870), held it to represent an ancestor of the 



FIG. 157. Zoaea larva of Crangon vulgar is, lateral view. (After Sars.) 

 Letters as in previous figure. 



Schizopoda and Decapoda ; others, like Glaus (1878), pointed out that 

 such a conclusion would imply that in the ancestral Decapod the 

 hinder part of the thoracic region was unsegmented, and that these 

 segments, when they did appear, must have been secondarily inter- 

 calated. If such reasoning were justified it would sever the hio-her 

 Crustacea from all connection with the lower Crustacea, for in these 

 latter the segments follow one another in development in un- 

 interrupted series from before backwards ; and so the conclusion was 

 drawn that the Zoaea had no significance whatever. 



Balfour then pointed out that in the more primitive types of 

 Zoaea the posterior thoracic segments, although very thin, are distinctly 

 present, and therefore surmised that the Zoaea might represent in 

 a modified form the ancestor of higher Crustacea. On the principles 

 laid down in this book, we must agree with him. The Zoaea was 

 clearly a larval form in the life-history of an ancestor common to the 

 Schizopoda and Decapoda, in a word, we might say, to the primitive 

 Malacostracan, and therefore represented an ancestor of this Mala- 

 costracan. But in that case, what stage in the evolution of the 



