148 



INVERTEBRATE ANIMALS. 



Crustaceans, whicli occur in both fresh and salt water. They are 

 distinguished by the fact that the body is entirely enclosed in a 

 shell, which is made up of two lateral halves or valves. The valves 



of the shell are united by a 

 membrane along the back, but 

 can be opened below, so as to 

 allow of the protrusion of the 

 feet. 



Very closely related to the 

 Copepods is a group of curious 

 Crustaceans, which are often 

 called " Fish-lice " {Ichthyoph- 

 thira), because in their adult 

 condition they are attached 

 parasitically to the skin, eyes, 

 or gills of fishes. The males, 

 however, are usually free. 



In its parasitic condition (fig. 

 98), the animal is more or less 

 swollen and deformed, its limbs 

 being often rudimentary, and 

 the segmentation of its body 

 obscured or lost. They are 

 firmly fixed, in various ways, 

 to their hosts, and the females 

 carry large external bags of 

 eggs (ovisacs). The young of 

 the rish-Iioe, however, have well-developed limbs and eyes, and 

 lead an active locomotive existence. 



Fig. 98. — Female of Achtlieres Carpenteri, 

 iiiagnifierl. Tlie line placed alongside uf 

 the figure shows the real size, a Ab- 

 domen ; d Disc of attachment developed 

 upon the last pair of thoracic limbs ; u 

 Ovisac. 



Order Cirripedia. 



The last order of Cncstacea is that of the Cirripedia (Lat. cin-as, 

 a curl ; and pe.i, foot), comprising the so-called Barnacles and Acorn- 

 shells, both extremely unlike Crustaceans to look at. All the Cirri- 

 jjedes are distinguished by the fact that, whilst they are quite free 

 when young, and very similar to some of the little Crustaceans just 

 desci-ibed, v,]\en adult they ai'e immovably fixed by their heads to 

 some solid object. In this fixed condition the body and internal 

 orgiins are, in most cases, protected by means of a calcareous shell, 

 composed of many pieces, and the only part of the body which 

 remains movable is the legs, which are constantly thrust out of the 

 shell and again drawn in in quest of food. The Cirripedia were 

 formerly described as " multivalve " shell-fish (MoUusea), owing to 



