192 



Arihropoda. 



in large blood reservoirs^ whence it is passed on to the gills. After 

 receiving fresh supplies of oxygen^ it travels through special vessels 

 to the pericardium, enters the heart through the ostia, and then 

 recommences the circulation. 



Excretory organs. Two pairs of tubular organs, probably 

 representing the segmental organs of the Annelids, are found in the 

 Crustacea. The inner opening is wanting ; they are usually of con- 

 siderable size and much coiled. The foremost pair, the antennary- 

 glands, open on the basal joints of the second antennae; the 

 second, the shell-glands,* at the base of the second maxilla. 

 Both pairs are seldom developed in the same animal ; frequently one 

 pair is present, and atrophies later, when the other is formed.t Often 

 both are absent. 



The great majority of Crustacea are of separate sexes. The 

 genital organs open ventrally, as a rule at a considerable distance 

 from the posterior end, and usually by two distinct openings. The 

 apertures of the oviducts are, in many forms, further forwards than 

 those of the vasa deferentia. Not infrequently, parthenogenesis 

 occurs (see Branchipus, Apus, the Daphnids). 



The development of the Crustacea is usually connected 

 with a very distinct metamorphosis, for the young one, 



when it leaves the egg, 

 is essentially different 

 from the adult. The 

 difference depends, 



amongst other things, 

 upon the smaller number 

 of segments and limbs 

 possessed by the larva ; 

 and further upon the 

 different structure and 

 even function of these 

 limbs. A large number 

 of Crustacea leave the 

 egg in the so-called 

 n a u p 1 i u s-state ; as 

 small, compact creatures, 

 furnished only with the 

 first and second antennae 

 and the mandibles. These 

 appendages are all de- 



Fig. 153. Naupliua of Fenasus. 

 After Pr. Miiller. 



Enlarged.- 



* TUs name is connected with the fact that the glands often {e.g., in Apus) lie for 

 the most part in the carapace. 



t Among the Entomostraca, the shell-gland is vmiversaUy present 

 in the adult, the antennary gland in the larva; whilst, conversely, the adult 

 Malacostracan exhibits the antennary-gland, the larva, the shell-gland. 



