536 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.49. 



the Malacostraca have generally developed special gills in hranchial 

 chambers on the thorax or in connection with the appendages. The 

 Crustacea have clung to the gills with great tenacity and instead of 

 acquiring some other method of respiration have in many cases 

 evolved elaborate accessory structm'es to facilitate the proper 

 operation of the gills, as in Calappa, Emerita, etc. It is interesting 

 to note that none of the primitive crustaceans have become terrestrial. 

 Those which have taken up their abode on land have usually been 

 able to do so through the further modification of already specialized 

 respiratory organs. Some terrestrial Malacostraca fill the branchial 

 cavity with water before going on land, others have labyrinthine 

 cavities which serve as lungs developed from the walls of the branchial 

 chambers (Birgus, TlialassiTia) ; the land amphipods have lost the 

 thoracic appendicular gills and breathe through the general body 

 surface; the isopods breathe through their flattened pleopods and 

 some have even developed tracheal tubes like those of insects on 

 these appendages. It would have made a great difference in the 

 racial history of the Crustacea if they had hit upon tracheae early, 

 instead of recently in one of the groups already adjusted to a terres- 

 trial existence. Here as everywhere in the course of organic evolution 

 success has brought its penalty. Branchial respiration made crusta- 

 ceans so successful in aquatic habitats that the race got in a rut and 

 became so set that it could not easily adjust itself to land. This 

 limits crustaceans to water or to moist situations; comparatively 

 few have been able to adjust their respiratory processes so as to take 

 their oxygen directly from the air. They are also thus exposed to 

 the dangers which accompany the fouling or drying up of water. 



In their reproductive activities the Crustacea have one peculiarity, 

 which has already been mentioned, the habit of carrying the eggs 

 and young during part of their development. This, with the fact 

 that most eggs or embryos witliin the eggshell are quite resistant to 

 dessication, makes possible adjustment of reproductive habits to 

 fresh water or land. The prawns in fresh water (MacrohracMum) 

 differ little from their relatives in the ocean (Palaemon). Many 

 fresh-water crustaceans, however, carry the young longer than 

 marine species of the same type (Astacidae), and in some the young 

 leave the parent in practically the adult form (Potamonidae). Yet 

 some of the most truly terrestrial species go back to the ocean to 

 hatch their larvae (Coenohita, Birgus). When we consider the 

 remarkable powers which the eggs of many marsh and pool inliabiting 

 species have of resisting extreme temperature and dessication 

 (BrancMpus, Diaptomus, Cypris), it seems strange that no entomos- 

 tracans live on land. In general there appears to be nothing about 

 the reproductive activities of Ci'ustacea which would prevent ready 

 adjustment to land habitats. 



