104 AXATOMV AND DEVELOPMENT OF THE ECHINODEKMS 



logical knowledge alone. A more exact knowledge of development 

 involved the necessity for, and at the same time furnished the key to, 

 a more accurate idea of the adult structure of the Echinoderms. 



The ordinary Echinoderms sufficiently try the patience of the 

 anatomist ; and any one who has ever endeavoured to dissect a Holo- 

 thuria, must recollect the feeling of despair with which he regarded 

 the knotted, glairy eviscerated mass, which was too often the reward of 

 all his care and caution. Undaunted by the great practical difficulties, 

 however, Prof Mialler has entered into these complementary investi- 

 gations (which are contained in the fourth and fifth treatises of the 

 foregoing list) ; the errors, difficulties, and contradictions which for- 

 merly infested the subject have been cleared up and rectified, and the 

 structure of the Ophiuridse, Asteridje, Echinidae, and Holothuriadse is 

 now capable of being reduced to broad general propositions. Without 

 by any means claiming for the celebrated Berlin physiologist the 

 merit of discovering facts of organization, due to Tiedemann, to 

 Valentin, to Krohn and others, it yet cannot be denied, that under his 

 hands these facts have first assumed their due importance, and become 

 moulded into a consistent whole. Under his authority, then, without 

 ■always caring to indicate the original sources of information, we shall 

 give the following summary of some points of the organization of the 

 Ophiuridje, AsteridjE, Echinidae, and Holothuriad^, as preliminary, 

 and indeed necessary, to a proper comprehension of their genetic 

 phaenomena. 



It is not, however, necessary for our present purpose to enter 

 upon the anatomy of any other systems of organs than the water- 

 vascular system, the blood-vascular system, and the nervous 

 system. 



In all the families cited, the fundamental part of these three systems 

 consists of three distinct rings, surrounding the oesophagus ; the blood- 

 vascular ring lies innermost, the water-vascular ring next, the nervous 

 ring outermost. 



The blood-vascular ring, besides the branches which it gives off, is 

 always connected with two vessels which run along opposite sides of 

 the intestine {Holothiirid) ; and in Asteridae and Echinidae there is a 

 distinct tubular heart which connects the vascular ring round the oeso- 

 phagus with another vascular ring surrounding the anus, from which 

 branches pass to the ovaria, &c. 



Branches are given off from the principal blood-vascular ring 

 towards the ambulacra, and in the Holothuriada; it appears very 

 probable that these branches accompany and indeed inclose the 

 nerves. 



