330 LECTURE XV. 



different species increase, and the clearer becomes our conception of 

 the unity of the general plan which pervades their organisation. 

 This is especially the case with regard to the phenomena which we 

 recognise in tracing the development of particular animals. But, 

 before entering on the stages of the development of the individual 

 Crustacean, I may premise a few remarks on the phases which the 

 class itself has passed through from the period when we first re- 

 cognise its representatives among the animals that have heretofore 

 peopled our planet. 



To render my remarks on the paleontology of the Crustacea more 

 intelligible, I may repeat that the class is naturally divisible into 

 two great groups ; but not, as Leach and Macleay believed, according 

 to the sessile or pedunculate character of the eyes, for you find eye- 

 stalks in Entomostracans — to use the language of Leach, "podoph- 

 thalmatous " species may be found amongst the lowest Phyllopoda, 

 e. g. in the genus Brancliipus, as well as in the highest members 

 of the class. We had to seek, therefore, for some more constant and 

 important character for the primary division of the Crustacea, and 

 this character we found in the constancy of the number of the seg- 

 ments composing the body in certain members, and the variability of 

 the number of segments in the rest of the class. In the majority of 

 the existing species, including the typical members of the class, the 

 thorax includes seven segments, the abdomen seven segments, and 

 seven are likewise indicated by the appendages in the head, twenty- 

 one being the archetypal number of the segments in the division of 

 the Crustacea, called " Malacostraca ; " at all events, the number 

 seven is constant with regard to the thoracic segments, and is never 

 exceeded in the abdominal segments in this sub-class. In the 

 lower sub-class, the number of segments varies, being often more 

 and sometimes less than seven. 



The Malacostraca, again, were divided according to the relative 

 length of the thorax and abdomen, or according as they have " long " 

 or "short" tails, as the latter part is commonly called ; in accordance 

 with which, the technical names of these subdivisions, or orders, 

 have been devised. The lobster is the type of the Macroura, or 

 long-tailed order, and the crab of the Brachyiira, or short-tailed 

 Crustaceans. But Nature never moves per saltum, and there is a 

 little group which has an intermediate proportion of the abdomen, 

 and it has been called '^ Ajio?)ioitra." 



All the three orders are represented in the tertiary formations and 

 in their pliocene, miocene, and eocene divisions ; but no species of 

 crab, or brachyurous Crustacean, has hitherto been met with in 



