XXXll INTRODUCTION. 



The propagation of the Crustacea proper is invariably 

 oviparous, and the sexes are distinct. The reproductive 

 organs in either sex are double, the two elements being 

 perfectly similar, and occupying a corresponding position 

 on each side of the median line. The two are wholly 

 independent of each other, having no communication 

 even to the efferent opening, there being one of them to 

 each. Dr. Milne Edwards mentions the following curious 

 fact: — " Cette independance des deux moities de I'appareil 

 de generation est si complete qu'on a vu un cas, ou I'un 

 des cotes etait male et I'autre femelle, sans que cette 

 monstruosite eut entraine aucune autre perturbation sen- 

 sible dans la conformation de ces organes."* They are 

 very similar in arrangement, position, and general relation 

 to the other organs in the two sexes. 



In most cases the eggs are carried by the female until 

 they are hatched; but in some they are previously de- 

 posited in the sand. In different families the eggs are 

 carried by the mother attached to different parts of the 

 body. In the Decapoda they are borne on the under 

 side of the abdomen, attached to the abdominal false 

 feet. In the genus Mysis, a pouch is formed at the base 

 of the posterior thoracic legs,t in which the eggs remain 

 until the young are excluded. In Thi/sauopoda, another 

 genus of the Mysidae, they are contained in two oval 

 purses, depending from the same part.J 



* Hist, des Crust, t. i. p. 165. t See p. 336. J See p. 346. 



