BIVALVED GROUP 



3249 



ture firmly to its point of attachment. Hence it follows that the fixed end of the 

 stalk is the front extremity of the body. In the allied stalkless barnacle (Mega- 

 lasma) the shell is attached directly to the support. We are thus led on to the acorn 

 barnacles (Balanus), in which the entire animal is inclosed in a shell formed 

 originally of six pieces, which grow into a tube of variable length. Some of the 

 latter group (Balanidce), namely, the genus Coronula or coronet barnacles, attach 

 themselves to the skin of whales. The burrowing barnacle ( Tubicinella) has the 

 same instinct. When adult it is long and cylindrical, consisting of a stout, stony 

 rod, marked with a series of annular ridges. This is buried deeply in the skin of 

 whales, sometimes penetrating as far as the blubber. 



These Cirripedes are not true parasites, inasmuch as they do not extract nour- 

 ishment from the animal to which they are attached; but many members of the 

 group live exclusively upon other living beings, and nourish themselves at their ex- 

 pense. One form, for instance, Proteolepas , is in the adult condition a maggot- 

 shaped, limbless, shell-less body, living within the mantle chamber of other mem- 

 bers of the same order; while the root-headed Cirripedes {Rhizocephala) live parasit- 

 ically upon the higher crustaceans. They are degenerate forms, possessing neither 

 appendages nor segments, the body being a mere sac, devoid of alimentary canal, 

 and absorbing nutriment by means of the root-like processes branching throughout 

 the body of the host. 



BIVALVED GROUP Order OSTRACODA 



This order is a small assemblage characterized by the possession of a bivalved 

 shell, formed from the right and left halves of the carapace, and furnished with an 

 elastic hinge to separate the valves and a muscle to 

 keep them shut. The shell incloses the body which 

 is unsegmented, has a rudimentary abdomen, and bears 

 seven pairs of appendages, namely, two pairs of an- 

 tennae, three pairs of jaws each belonging to the head, 

 and two of limbs attached to the thorax. These limbs, 

 however, are stout and narrow, and, as a rule, there 

 are no special respiratory organs. Ostracods occur 

 both in fresh water and the sea; the best-known forms 

 being Cypris and Cythere. The former contains species 

 found in ditches and ponds in England. When the 

 waters in which they live dry up, the species of Cypris 

 bury themselves in the mud until rain falls; the eggs, 

 which are spherical, being attached to aquatic plants, 

 mostly marine, haunting rocky pools on the coast and crawling among the seaweed. 

 In Cypridina, on the contrary, which is also marine, the animals dart about with 

 velocity; the females carrying their eggs between the valves of the shell attached to 



their feet. 

 204 



COPEPODS. 



a. Female Cyclops, with egg sacs; b, 

 c. Nauplius and later larva of 

 same. 



The species of Cythere are 



