258 ThompsorCs Zoological Researches 



they are extremely quicksighted and wary, darting away or 

 descending, tail foremost, when any attempt is made to cap- 

 ture them ; they are omnivorous, seizing and eating every 

 animal substance which the current or the tide carries along 

 with it, and contending like vultures for the possession of 

 the larger masses. When confined together in a vessel of 

 sea water, they will even act the cannibal, killing and devour- 

 ing one another 5 a fact in their history to which the writer 

 of this notice has been a witness. As they generally swim 

 near the surface, they there enact the part of nature's sca- 

 vengers, and remove thence all extraneous matters fitted to 

 their digestion ; while their allies, the true shrimps, are per- 

 forming a similar office at the bottom, or in the sand. 



The most singular part of the economy of the My sis, how- 

 ever, is its mode of generation. The female is provided with 

 a pouch placed beneath and just behind the thorax, and co- 

 vered by valves of curious workmanship to shut and open. 

 Into this pouch the eggs are received when excluded from 

 the ovarium ; where, enveloped in a mucous or glairy secre- 

 tion, they are hatched and gradually matured, without any 

 visible attachment to their parent. The developement of the 

 embryo appears to be quick, for each female has several 

 broods in the season ; and the evolution of the foetus is 

 *' simply a gradual developement of parts : " a proof that all 

 Crustacea do not undergo a metamorphosis ; and a fact worth 

 remembrance, were it only as a caution to the naturalist 

 when he feels disposed to carry his generalisations beyond 

 actual observation. The period of gestation being elapsed, 

 the parent spreads open the valves of its pouch, " when the 

 whole brood emerge at once into the ambient element, and, 

 in most of the species, continue associated with the community 

 from which they sprang. This curious and extraordinary 

 piece of economy can hardly fail to be regarded by the phy- 

 siologist as equally interesting with that of the opossums and 

 other marsupial quadrupeds, and of a much more unaccount- 

 able nature; for in these last, although the object of the 

 Creator is not obvious, yet we can understand the manner in 

 which it is carried into effect : the young being excluded from 

 the uterus when they have scarcely attained a fourth part of 

 the growth of the embryos of other animals, naked, help- 

 less, and blind, they are received into the abdominal pouch of 

 the mother, and, by some wond^erful instinct, or by the mother's 

 agency, attached each to one of the teats which are situated 

 within it ; from whence, when sufficiently grown, they make 

 occasional sallies, until able entirely to provide for themselves. 

 In the opossum shrimp, on the contrary, we comprehend the 



