ECHINODEKMATA. 161 



(429.) The Trichina spiralis (fig. 79) is an Entozoon found in im- 

 mense numbers imbedded in the cellular intervals between the mus- 

 cular fibres ; and in some instances all the voluntary muscles seem full 

 of these creatures, exhibiting, when viewed with the naked eye, an 

 appearance imitated in the annexed figure (fig. 79, c). On examining 

 the white specks attentively under the microscope, every one of them 

 is seen to be a flask-shaped vesicle, apparently formed of condensed 

 cellular membrane, in which the minute animal is lodged ; and when 

 this outer covering is ruptured, as at a, the worm escapes. A mag- 

 nified view of the Entozoon is given at 6, coiled up in the position in 

 which it is seen prior to the destruction of the sac that enclosed it. 

 The body seems to be filled with granular matter, which escapes when 

 the worm is torn asunder (d) ; but whether it possesses a true ali- 

 mentary tube is not as yet satisfactorily determined. 



CHAPTER VIII. 



ECHINODEKMATA* (Cuv.)- 



(430.) THE next class of beings which presents itself for our con- 

 sideration seems, upon a partial survey, to be completely insulated and 

 distinct from all other forms of living creatures ; so peculiar is the 

 external appearance and even the internal organization of the families 

 composing it. The casual observer who should, for the first time, exa- 

 mine a star-fish or a sea-urchin, two of the most familiar examples of 

 the ECHINODERMATA met with upon our shores, would, indeed, find it a 

 difficult task to associate them, either to the simpler animals we have 

 already described, or to more perfect forms of existence hereafter to be 

 mentioned ; they would seem to stand alone in the creation, without 

 appearing to form any portion of that series of development which we 

 have hitherto been able to trace so continuously. 



(431.) But this apparent want of conformity to the general laws of 

 development vanishes on more attentive examination ; so that we may 

 not only follow the steps by which every family of this extensive class 

 merges insensibly into another, but perceive that, at the two opposite 

 points of the circle, the ECHINODERMATA are intimately in relation with 

 the POLYPS on one hand, while on the other they as obviously approxi- 

 mate the Annulose animals, to which the most perfectly organized 

 amongst them bear a striking resemblance. 



(432.) It would be impossible within our present limits to do more 

 than lay before the reader the most important types of structure 



a hedgehog ; ep/ia, the skin. 



