On Excretory Organs &c. in certain Isopoda. 415 



Contrasted with other regions, the richness of Shetland in 

 cup-, apple-, and turnip-sponges, its swarms of Arachnactis 

 alhida and Echinus norvegicuSj the occurrence of Cidaris 

 papillafa, the huge Cucumaria frondosa, Eurythoe horealis, 

 Lcetmatonice, the Ampharetidge, the peculiar mollusca, the 

 large size and frequency of the torsk, the presence of Chimmra 

 and its many Cetaceans, give it characters of its own. The 

 majority of these appear to have come from the north and 

 east. Forms, again, which occur in deep water in Shetland 

 appear between tide-marks in the Channel Islands, and occa- 

 sionally in diminished bulk. The presence of Mediterranean 

 forms — Xanflio rivulosusj Pagurus tuberculatus, and Pinna 

 rudis — show how difficult it is to explain the centres of origin 

 or the lines of migration. 



Boldly mapj)ing out the warmer southern area is the distri- 

 bution of Noctiliicaj of the silky-spined urchins, the brightly 

 coloured Eurylepta, and Drepanophorus amongst Nemerteans, 

 of the southern sea- mouse (Hermione), of the crustaceans 

 Alpheus, the spiny lobster, Polyhius and Dromia, the moUusks 

 Gastrocltcena, Gahornma, Mactra glauca, Haliotis, and the 

 frequency of the cuttles between tide-marks, of Balanoqlussus, 

 the finely coloured wrasses, the red mullets, and the pilchards. 

 Almost all these forms are essentially southern, and they show 

 no stragglers leading northwards, such having probably been 

 checked more by the diminished annual temperature than by 

 the absence of favourable currents. 



In conclusion, limited as the area we have been considering 

 is, it is apparent that while some forms are common to all, 

 certain restraining influences check the spread of others, so 

 that they become more or less characteristic of the several 

 regions. Moreover, the mixed nature of the fauna shows 

 that we have to do with several sources of origin, some of 

 which date back to geological periods marked by a different 

 arrangement of the land, and a consequent change in the 

 temperature of the water. 



LX. — On Excretory Organs and Sexual Conditions in 

 certain Isopoda. By BOHUMIL Nemec, of Prague *. 



In the course of my investigations upon Isopods I have 

 arrived at certain interesting results relating to their excre- 

 tory and genital organs, of which I here furnish a short 

 rSsumS. 



* Translated by E. E. Austen from the ' Zoologischer Anzeiger,' 

 Bd. xix. no. 507 (July 6, 189G), pp. 297-301. 



29* 



