296 LECTURE XIV, 



LECTURE XIV. 



CRUSTACEA. 



The class of invertebrated animals which forms our present subject 

 may be regarded, from one point of view, as having been entered upon 

 in the preceding Lecture ; for, were we cognisant of the Lerna^ans 

 and Cirripeds in no otlier than their locomotive state, we might be 

 led, with those naturalists who judge of an animal from one of its 

 vital stages only, to classify them with the Crustacea, and the pupal 

 barnacles more especially with the bivalve Entomostracans. All 

 Crustacea, however, differ from the Epizoa in being branchiferous ; 

 and from the Cirripedia in being dioecious, with well-developed males. 

 And the subjects of the preceding Lecture must have sufficiently 

 impressed us with the necessity of careful investigation of problematic 

 species at every stage of their development, in order to determine 

 their affinities and position in the scale of nature. 



In tracing the progressive march of structural complications in the 

 animal kingdom, we see that the organs of vegetative life much more 

 rapidly acquire their full development than those of animal life ; 

 and, in reference to the latter, we find a more regular gradation of 

 perfection as the classes ascend in the scale. Take the Articulate 

 sub-kingdom for example. The Entozoa may show a complete alimen- 

 tary canal, with mouth and vent, and with salivary and biliary coeca: 

 they may have the sexes separate, and the male and female organs 

 marvellously developed ; yet be devoid of every trace of a locomotive 

 organ, and have a smooth, soft, continuous, lubricous integument. 

 In the Annulata we found the integument firmer, but alternately 

 harder and softer, so as to be divided into segments with flexible 

 articulations; and in the higher forms of this class, each of the joints 

 was provided with distinct limbs, although these presented the lowest 

 form of such, e. g., as simple tubes including protractile setae. In the 

 larval Ejjizoa we saw the limbs presenting a true jointed structure, 

 but lost or becoming more or less abortive in the fixed adults; whilst 

 in the Cirrijiedia several pairs of jointed limbs are retained in the 

 adult state, but their rapid actions are subservient to the acquisition 

 of food, not to locomotion. 



We make the easiest and most natural transition from the lower 

 forms of Articulata to the Crustaceous class., by passing to it from 



