498 CRUSTACEA MALACOSTBACA. 



gives off arteries directed anteriorly and posteriorly. The 

 branchiae are elongated and flattened sacs in relation with the 

 bases of the thoracic limbs, as above described. The blood 

 from them returns direct to the pericardial sinus where it mixes 

 with venous blood from other parts of the body. In the species 

 of Orchestia and Talitrus which live in moist situations on 

 land, the branchiae are spirally twisted. In the Cyamidae 

 they are long and tubular. 



The generative organs are tubular and much alike in the 

 two sexes, but the ducts of the male open, as usual, on the 

 eighth, those of the female on the sixth thoracic segment. The 

 former are situated on papillae, and abdominal feet are not 

 modified, as in some Isopods, as copulatory organs. Nebeski has 

 described a remarkable condition of the generative gland in 

 the male Orchestia. The posterior part produces spermatozoa, 

 but the anterior part consists of primitive ova, which however 

 do not become functional. 



The males are distinguished from the females not only by 

 the absence of oostegites, but by the stronger development of 

 the prehensile thoracic feet,, and by the shape and sensory 

 structures of the antennae. 



The spermatozoa are filiform. 



The eggs undergo their development in a brood pouch beneath 

 the thorax of the female, being covered by the lamellar 

 oostegites (2-4 pairs). 



The yolk sometimes (Gammarus locusta and other marine 

 species) undergoes a complete segmentation. Sometimes (G. 

 pulex), after a superficial segmentation, a peripheral cell layer 

 is formed, which secretes a delicate larval membrane beneath 

 the egg membrane. A ventral primitive streak is then formed, 

 and on the dorsal side a peculiar globular organ arises by in- 

 vagination, with an orifice which has been erroneously taken 

 for a micropyle. This is the first rudiment of a structure, 

 known as the cervical gland (dorsal organ), which is confined to 

 embryonic life. It is regarded by Grobben as homologous with 

 the cervical gland of Branchiopods. The appendages are deve- 

 loped from before backwards, and the body of the embryo is 

 ventrally flexed, the reverse of the position in Isopods. The 

 young animals usually possess all their appendages at hatching, 

 and in all essential external points have the structure of the 



