21 



sometimes they perforate the corresponding segment of the sternum. In the 

 female they usually open, near the middle line of the body, on the sternum cor- 

 responding mth the ante-penultimate pair of legs, but occasionally they perforate 

 the coxopodites of these appendages themselves. 



The ova are impregnated internally, the organs of insemination being the 

 modified 1st and 2nd pairs of abdominal appendages of the male. Of these the 

 1st pair are the largest, and are made up of a propodite followed by a long 

 rigid exopodite which is rolled like a scroll to form a hollow tube of considerable 

 length. The 2nd pair are slender and consist of a propodite followed by a 

 styliform endopodite which is either closely applied to the preceding appendage 

 or is actually lodged in its channel. 



The extruded ova, which are small and very numerous, are attached to the 

 setose abdominal appendages of the female until they are hatched. 



In all the Brachyura except a few species of land-cralis the young leaves 

 the egg as a zoasa. 



In the typical crab-zoa?a the body consists of a cephalothorax enclosed in 

 a carapace, followed by a slender six-jointed abdomen that terminates in a fork. 



The carapace is usually armed with toui- large spines, namely, one pointiu"- 

 forwards and corresponding with the rostrum, one very large one pointing back- 

 Avards, and one on either side. 



The abdomen is without appendages, but the following cephalothoracic 

 appendages are present : — large stalked eyes, uniramous antennules, non-flagel- 

 late antenna? provided with an antennal scale, Uvge mandibles consistino- of 

 propodite only, two pairs of maxilky differing little in composition from those 

 of the adult, and two pairs of large biramous maxillipeds which are the prin- 

 cipal organs of locomotion at this free-swimming stage. 



In the course of a series of moults the remaining appendages are gradually 

 acquired and at the last moult the zoa?a changes into a Merjalopa. 



In the Megalopa the cephalothorax is much broader than in the zoxn and its 

 spines are usually much reduced in size. The abdomCn though relatively shorter 

 than in the zofea is still of considerable size, and though curved is carried out 

 behind the cephalothorax : it consists of 7 segments and the terminal fork is 

 replaced by an oval telson, and the abdominal appendages are present. 



By a further series of moults the Megalopa gradually changes into the adult 

 form. 



0. Of flic Braclujurovs Carcqjace. 



In the typical crabs the carapace is a transversely-oval shield with sharply 

 defined lateral borders that separate the dorsal surface from the sidewalls, anil 

 marked by certain mtiires and groores which delimit definite areas or regions. 



