The disposition of these caeca, marking as they do the morphological position of fore-, mid-, and hind-gut, is of peculiar interest owing to the variations exhibited. From some unpublished drawings of Mr. E. H. Schuster, which he kindly lent me, it appears that in certain Decapods, _e.g._ _Callianassa subterranea_, the length of the mid-gut between the anterior and posterior caeca is very long; in _Carcinus maenas_ it is considerable; in _Maia squinado_ it is greatly reduced, the caeca being closely approximated; while in _Galathea strigosa_ the caeca are greatly reduced, and the mid-gut as a separate entity has almost disappeared. The relation of these variations to the habits of the different crabs and to their modes of development is unknown. The =reproductive organs= usually make their appearance as a small paired group of mesodermal cells in the thorax comparatively late in life; and neither in their early development nor in the adult condition do they show any clear signs of segmentation or any connexion with a coelomic cavity. The sexes are usually separate, but hermaphroditism occurs sporadically in many forms, and as a normal condition in some parasitic groups (see pp. 105–107). The adult gonads are generally simple paired tubes, from the walls of which the germ-cells are produced, and as these grow and come to maturity they fill up the cavities of the tubes; special nutrient cells are rarely differentiated, though in some cases (_e.g._ Cladocera) a few ova nourish themselves by devouring their sister-cells (see p. 44). The oviducts and vasa deferentia are formed as simple outgrowths from the gonadial tubes, which acquire an opening to the exterior; they are usually poorly supplied with accessory glands, the epithelium of the canals often supplying albuminous secretions for cementing the eggs together, while the lining of the vasa deferentia may be instrumental in the formation of spermatophores for transferring large packets of spermatozoa to the female. In the vast majority of Crustacea copulation takes place, the male passing spermatophores or free spermatozoa into special receptacles (spermathecae), or into the oviducts of the female. The spermatophores are hollow chitinous structures in which the spermatozoa are packed; they are often very large and assume characteristic shapes, especially in the Decapoda.