CRUSTACEA. 227 



occur. Of modes of respiration, there are many grades, — 

 by the general surface, by currents of water in and out of 

 the posterior part of the food-canal, by thin plates on the 

 legs, by well-formed gills. We miss the numerous excretory 

 nephridia of Annelids ; the green-glands of lobsters, etc., 

 probably represent a pair ; the shell-glands of Fhyllopods 

 and Copepods and some other structures seem to be in part 

 at least excretory. It is possible that shell-making is an 

 organised method of getting rid of some waste-products. 

 There are many peculiarities connected with reproduction : 

 thus parthenogenesis for prolonged periods is common 

 among " water-fleas " ; hermaphroditism occurs in barnacles, 

 acorn-shells, etc. ; the hermaphrodites are sometimes accom- 

 panied by pigmy " complemental " males ; the two sexes are 

 often very diverse, and the females of lower forms often 

 carry the males about with them. The spermatozoa are 

 usually exceptional in being very slightly motile. Some 

 appendages are often modified for copulation or for carry- 

 ing the eggs. The ova contain a considerable quantity 

 of yolk, and the segmentation is always peripheral or 

 centrolecithal, that is to say the core of yolk does not 

 divide into cells, as the peripheral formative protoplasm 

 does. 



Life-History. — In the development of Crustaceans, there 

 is almost invariably some metamorphosis, and often a very 

 marked one. In other words, the larva hatched from 

 the egg is rarely like the parent, and only acquires the 

 adult characters after a series of profound changes. In some 

 cases {Nebalia, Mysis) a metamorphosis takes place within 

 the egg-cases, and in the few forms in which development 

 seems to be direct slight traces of metamorphosis are 

 found; 



Almost all Entomostraca (lower Crustaceans) and the 

 higher Crustaceans Euphausia and Penaus are hatched in a 

 Nauplius stage, — with an unsegmented body, with three 

 pairs of appendages of which the two posterior pairs are 

 biramose, and with a median eye. The three pairs of append- 

 ages become the first and second pairs of antennae and the 

 mandibles of the adult. The head-region of the Nauplius 

 becomes the head-region of the adult, the posterior region 

 also persists, the new growth of segments and appendages 



