132 AN INTRODUCTION TO ZOOLOGY. 



gated form, consisting merely of ectoderm and endoderm 

 surrounding a median cavity. This larva, called a planula, 

 subsequently acquires a moutli and tentacles. 



PZctce in classification. Sub-kingdom CcBlenterata ; class 

 Htdrozoa ; order HydromeduscB ; sub-order JEleutherMastecB ; 

 family Hydroidece; genus Hydra. 



Detailed description. See Huxley and Martin's " Practical 

 Biology"; Parker's " Elementary Biology " ; Howes' "Atlas 

 of Practical Elementary Biology." 



III. Type of the higher Vermes, which, together 

 with the remaining sub-kingdoms of the animal 

 kingdom, constitute the fourth grade of animal life, 

 triploblastie animals with a body-cavity surrounding 

 the alimentary canal. 



(The study of the third grade, represented by forms belong- 

 ing to the lower Vermes, is omitted here.) 



Vermes. Animals more or less resembling the Earth- 

 worm, Lumbricus (various species, terrestris, etc.). 



The Earthworm is a bilateral (p. 81) animal, with a body 

 divided into segments or metameres\(p. 83) ; it differs from 

 Hydra, and agrees with all the higher types of animal life, in 

 possessing a third body-layer, the mesoblast (p. 30), and a body- 

 cavity (pp. 40, 46) surrounding the alimentary canal. It has 

 no limbs, and exhibits a characteristic crawling movement, 

 typical of its sub-kingdom, and effected by the contraction of 

 successive regions of the well-developed muscular coat of the 

 body-wall, which consists of an internal longitudinal layer, 

 and an external circular one. (In the earthworm external 

 bristles aid in crawling by taking a firm hold on the ground, 

 but many worms are smooth.) The body has attained a much 

 higher stage of. differentiation than that presented by Hydra. 

 The following organs are present : — Nei-vous System, con- 

 sisting of a paired dorsal ganglion or " brain," united, by 

 connectives which pass round the pharynx, with a double 

 ventral longitudinal nerve-cord ; organs of special senses are 



