318 MR. WESTWOOD ON THE SUPPOSED EXISTENCE 



4. In the next place it is to be observed, that Mr. Thompson's figures and state- 

 ments relative to the gradual development of the Zoes are totally at variance with one 

 of the received principles of ecdysis, which may be thus stated. When an animal 

 undergoes a variety of moultings, attended by alteration in form or development of 

 organs, there is a gradual tendency towards the organization of the perfect animal. 

 Now Mr. Thompson expressly states that his large Zoes differed from the smaller 

 ones in the greater degree of development of all their organs. This therefore is pre- 

 cisely what would occur in case the large Zoes were perfect animals ; and it is pre- 

 cisely what would not take place if the subsequent state of the Zoe were a Crab. 



5. It is worthy of notice that there are several peculiarities in Zoea so evidently 

 partaking of the Macrourous type, that it is surprising that Mr. Thompson should 

 not have noticed that these characters present themselves in so complete a state of 

 development, when compared with the Macroura, as to negative the opinion that 

 these animals would ever become Brachyurous. The elongated tail, the rostrated 

 cephalothorax, but more especially the structure of the mandibles and two pairs of 

 maxillae, may especially be noticed. 



6. If, as we shall subsequently perceive, there be no pretence for doubting the 

 correctness of Rathke's researches upon the Cray-fish, which is clearly proved to 

 undergo no metamorphosis, I think we are fully warranted from analogy in consi- 

 dering that the other Decapods do not undergo metamorphosis. Mr. Thompson, in- 

 deed, seems inclined to consider that in such case the Cray-fish " can only be regarded 

 as one solitary exception to the generality of metamorphosis * ;" although he had pre- 

 viously given his opinion of the weight of analogy in the second number of his Re- 

 searches, by stating that " metamorphosis having been proved in a single instance 

 amongst animals so uniform in structure as the Homobranchia, we may safely infer 

 from analogy, as far as regards the particular tribe alluded to, that it is general." 



These six considerations induce me to adopt the opinion that no sufficient ground 

 has been shown by Mr. Thompson for supposing that a metamorphosis of Zoes into 

 Crabs takes place. 



Secondly, therefore, we will proceed to notice Mr. Thompson's statements relative 

 to the hatching of the young Zoes from a female of the common Crab, and which he 

 states took place under his own eye. It is much to be regretted that Mr. Thompson, 

 having such ample opportunity, did not dissect the ova in various states, so as to 

 ascertain in the most satisfactory manner the gradual development of the embryo, as 

 Rathke has done in the Cray-fish. The statement, although short, ha-s, however, 

 such sufficient precision, that we are compelled to believe either that (notwithstand- 

 ing whatever may be advanced to the contrary) the young of the common Crab are 

 Zoes, or that the latter are parasitic animals, which in some unexplained manner are 

 introduced in the embryo state beneath the abdomen of the Crab ; and if we consider 

 the large Zoes observed by Mr. Thompson to be perfect animals, there is some ground 



* Zoological Journal, No. xix. p. 383. 



