292 GENERAL CHARACTERS OF CIRRHOPODA. 



the continual vibration of which, currents are produced in the 

 surrounding water, that serve to bring food to the animal, and 

 a fresh supply of fluid for the aeration of the blood by means of 

 the gills, which are situated at the base of these appendages. In 

 this conformation, there is an evident analogy with many Crus- 

 tacea. Further, the mouth is furnished with lateral jaws, which 

 no Mollusca possess ; and the Nervous System consists of a double 

 cord, with a pair of ganglia in each segment of the body, pre- 

 cisely as in the Articulata in general. The most interesting 

 proof, however, that the Cirrhopoda belong to this division, is 

 derived from the history of their development. On their libe- 

 ration from the egg, they present a form much more analogous 

 to that of some of the Entomostracous Crustacea, than to that of 

 the adult animal, which they only acquire after a series of 

 metamorphoses. 



827. The young of the Balanus ( 830) is inclosed in a 

 bivalve shell, of which the valves are united by a hinge along 

 the back, and open along the lower margin, for the protrusion of 

 a large and strong anterior pair of prehensile limbs, provided with 

 an adhesive sucker and hooks, and of six pairs of posterior legs, 

 adapted for swimming. The form of the animal thus bears a 

 strong resemblance to that of the Daphnia ( 804) ; and its 

 movements, too, are similar, consisting of a succession of jerks. 

 A pair of compound eyes, supported on a peduncle, is seen on the 

 anterior and lateral part of the body. Nearly all these organs 

 are thrown off together ; and the animal assumes an entirely 

 different form and condition. It first attaches itself to a neigh- 

 bouring surface by a layer of calcareous matter, which is subse- 

 quently to become the base of its new shell ; the valves which 

 surround its body are gradually developed on its envelope or 

 mantle ; the new ciliated arms begin to grow within ; and the 

 eyes disappear, the animal being henceforth destitute of the visual 

 sense. The form of the young Lepas is different, having more 

 resemblance to that of the Cyclops (Fig. 496). The body is not 

 inclosed in a bivalve shell, but its back is covered by an expanded 

 shield, like that of Argulus (Fig. 498) ; it has a single eye, not 

 stalked ; and three pairs of members, of which the anterior is 



