28o 



CRUSTACEANS. 



phkonima (enlarged three times). 



exceeding an inch in length. As in Cystosoma, the second pair of antennas are 

 obsolete; the head is large, with the eyes placed upon its summit. There are 



seven pairs of large thoracic appendages, the third 

 from the end forming a large and strong pincer. 



The species are widely distributed, although 

 most abundant in the tropics. Like many pelagic 

 animals, they are translucent, and mostly live in the 

 mantle - cavity of the ascidians Pyrosoma and 

 Doliolum, where the eggs are laid, and the young 

 hatched. 



To a certain extent connecting the Malacostraca 

 with the Entomostraca is a group of Crustaceans 

 known as the Leptostraca, and containing the three recent genera Nebalia, 

 Nebaliopsis, and Paranebalia, and a number of fossil forms. The affinities of 

 the group seem to lie with the Plryllopods on the one hand, and the Schizopods 

 on the other. The body is laterally compressed, and the whole of the cephalothorax 

 and the first four segments of the abdomen enveloped in a carapace, which springs 

 from the head, and is formed of two movable valves, closed by a muscle. Although 

 the eight thoracic segments are overlapped by the carapace, they are distinct and 

 movable. The abdomen consists of eight movable segments, or two in excess of the 

 normal number ; but there are only nineteen pairs of appendages. The head bears 

 a small, movable rostrum, and a pair of stalked eyes. The two pairs of antennas 

 are well developed, and there are three pairs of jaws. The appendages of the thorax 

 are foliaceous. The members of this group are marine, and widely distributed, 

 being found in cold and warm latitudes. The female carries the eggs attached to 

 her thoracic feet. 



Subclass Entomostraca. 



The Crustaceans of this division are small, and vary much more than the 

 Malacostraca, from which they differ in the following features. The number of 

 body-segments is not constant, but either greater or less than nineteen, and, as a 

 rule, there are no appendages to the abdomen. In the majority of cases the young 

 are hatched as a Nauplius. 



The Barnacles, — Order Cirripedia. 



The adult members of this group are so unlike typical Crustaceans that it can 

 hardly be a reproach to the older naturalists that they failed to discover their 

 affinity. Two well-known members of the order are the barnacles so frequently 

 attached to the bottoms of ships or floating timber, and the acorn -barnacles 

 covering the rocks on the coast. The barnacle (Lepas) consists of a tough longer 

 or shorter stalk, one end of which adheres tightly by means of a cement to the 

 timber or ship, while to the other is attached an oval compressed body encased in 

 pieces of shell, through two of the valves of which can be protruded six pairs of 

 slender, bristly, two-branched, filamentous limbs. These limbs, being the appendages 

 of the thorax, keep up a constant sweeping motion, whereby particles of food are 



