186 LECTURE XV. 



Brachyura the shorter abdomen or tail is so much more expanded in 

 the females as to cover the whole sternum, and render the sex dis- 

 tinguishable at a glance. 



In certain parasitic species of Cymothoa, the males have eyes, the 

 females, which are considerably larger^ have no eyes : they have pro- 

 bably been effaced by age and lack of use, as in the parasitic females 

 of the Epizoa. 



The lower forms of Entomostracous Crustacea, and especially the 

 suctorial species, have long been known to undergo remarkable 

 changes of form in repeated moultings during their growth, analogous 

 to the metamorphoses which have already been described in the 

 Epizoa : differing, in fact, from these only in degree ; the last or 

 procreative stage being attained in most Entomostraca under a 

 locomotive form, which differs less from that of the newly excluded 

 young, than does the corresponding mature stage of the deformed 

 parasitic Lernaea. There is a period when the young Limulus is 

 destitute of the characteristic caudal spine : this fact was first observed 

 by M. Edwards in the embryos on the point of exclusion from the 

 egg, at which period the abdomen supports only three pairs of ap- 

 pendages. 



With respect to the Malacostraca^ in 1778, a Dutch naturalist, 

 Slabber, described and figured a minute swimming Crustacean of the 

 genus called Zoea by modern naturalists : it was provided with a 

 pair of large and distinct eyes ; its carapace was armed with a long 

 frontal and a dorsal spine ; and its abdomen was terminated by a 

 forked tail. He preserved this little animal alive in sea water, which 

 was daily renewed, and on the fourth day he found that the animal 

 had changed its form ; the feet, eyes, and antennae were more 

 developed, the frontal spine had become comparatively small, the 

 dorsal one had disappeared, and the tail had changed from the 

 bifurcate to the spatulate form, and was fringed by a row of short 

 spines. Many years elapsed ere this observation was repeated, and it 

 seems to have been forgotten, when Dr. Leach, the most accomplished 

 Crustaceologist of his day, founded the principal character of the 

 class Crustacea on the absence of metamorphosis. 



During the spring of 1822, Mr. V. Thompson * captured an abun- 

 dance of the singular Zoeas In the harbour of Cove ; the largest of them 

 was daily supplied with fresh sea- water from May 14th until the 15th 

 of June, when it died in the act of changing its skin. The disengaged 

 members, invested with the new integuments, were changed both in 

 number and form, and corresponded with those of the decapod 

 Crustacea, the anterior pair being furnished with large pincers. Here, 

 therefore, was a strong indication that, under the form of a Zoea, was 



* Zoological Researches, vol. i. Part I. p. 1. 



