based on the Principle of Cephalization. 265 



general urosthenic, even the highest havmg a caudal disk for 

 attachment ; and in an up-and-down movement of the body in 

 locomotion, 3Iollusk-like, instead of the worm-like lateral move- 

 ment of the Annelids. The fact of this mode of movement has 

 been recently made known to the writer by Dr Wm. C. Minor, as 

 a distinctive feature of the Bdelloids. Quatrefages remarks that 

 the Planarians and Trematodes may well be regarded degraded 

 forms of the Hirudines, and the three tribes are arranged in one 

 group by Burmeister. — Third, GepJiyreans (of de Quatrefages), 

 or Holothurioid (Badiat e-like) Worms, including the genera, 

 Echiurus, Sipuncula, &c.* — Fourth, Cestideans, or Protozoic 

 Worms, including the Cestoids, in which there is no normal 

 digestive system, and the segments are independently self- 

 nutrient, f 



The orders of these classes of Articulates are the foUowinor : — 



o 



Alphatypic, 



Betatypic, 

 Gammatypic, 

 a Degradation al, 

 h. 



Insecteans. 



Insects. 

 Spiders. 

 Myriapods, 



Crustaceans. 



Decapods. 



Tetradecapods. 



Entomostracans. 



Worms. 



Annelids. 

 Bdelloids, 

 Gephyreans. 

 Cestideans. 



6. Subdivisions of the Orders of Insecteans and Cy^staceans 

 into Tribes. — (1.) The orders of Insecteans have each three divi- 

 sions, excepting that of Myriapods in which but two have been 

 recognised. The three of Insects are indicated on pages 83, 

 98. The fact that Insects are, in type-idea, /?/^??^ Articulates, 

 gives special importance to the wings in classification. The first 

 order includes the Prosthenics, in which the anterior wings are 

 flying wings, as the Hymenopters, Dipters, NeuropterSj Lepidop- 

 ters, and Homopters. The secotLcl consists of the Metasthenics or 

 Elytropters^ in which the anterior wings are not used in flying, 

 or but little so, as the Coleopters, Strepsipters, Orthopters, and 

 Hemipters. The Hemipters and Homopters, united in one tribe 

 by most entomologists, are hence profoundly distinct. The third 

 tribe, or Apters, embraces the Lepismids and Podurellids ; the 

 remaining Apterous insects being distributed among the other 



* The Holotliurioid characteristics are well exhibited by de Quatrefages in 

 Part ii. p. 248 and beyond, of " Kecberches Anatomiques et Zoologiques ftdtes 

 pendant nn voyage snr les Cotes de la Sicile," &c., in 3 vols, or parts, the 

 second by de Quatrefages. Paris. 



t The Aca?ithoce2yhalz, according to van Beneden and Blanchard, are ISTema- 

 toids (with which they agree in form and general structure), although with- 

 out a digestive system. Blanchard states that there is reason for believing 

 that the digestive system becomes atrophied with the growth of the animal, 

 and mentions that cases of like atrophy occur even in species of Gordius and 

 Nemertes. 



