CRUSTACEA. 115 



it constitutes an oval, compressed, shelly box with- 

 out a hinge, inclosing the whole animal, except at 

 one orifice where the head projects. 



A large number of this Order are parasitic upon 

 other marine animals, like the Epizoa; with which 

 they agree in another curious circumstance. The 

 females of many species in these groups carry their 

 eggs in two large, oval, cylindrical, or spirally 

 twisted bags (external ovaries), sometimes ap- 

 proaching in bulk that of the whole body, and 

 (in some of the Epizoa} often greatly exceeding 

 it in length. In Evadne, our only marine repre- 

 sentative of the Water-fleas (Daphnia, &c.), that 

 are so abundant in fresh waters, the eggs are depo- 

 sited in a sort of chamber in the upper part of the 

 shell, above the back, where the young remain, 

 even for several days after they are hatched. 



The lowest forms of all are the Podosomata, 

 which in many particulars approach the Spiders 

 so closely, that it is a matter of debate in which 

 Class they should be most naturally placed. I 

 have, however, followed Professor Milne-Edwards 

 in assigning them to the CEUSTACEA. 



Localities, &c. The Podosomata and the smaller 

 Entomostraca are found (the latter in great abund- 

 ance) in the tufts of sea-weeds that grow between 

 tide-levels, and the latter also swim in the open 

 sea, and may be collected with a Medusa-net. The 

 Epizoa and the parasitic Entomostraca dwell, as I 

 have already observed, on other animals, especially 

 fishes ; the former chiefly affecting the gills, or 

 gill-pouches ; the latter creeping on the surface of 

 the body : the particular prey of each species will 

 be mentioned in detail. The Cymothoada among 

 the higher forms have similar habits, but most of 

 i 2 



