44 



ECHINODERMATA. 



intestine, form a plexus along its first portion, 

 whose branches ultimately terminate in a large 

 longitudinal venous trunk (n, n, n, n). The 

 blood is conveyed from this great vein to the 

 right branch of the respiratory organ, (which 

 lies between the first and second portions of 

 intestine,) by a considerable number of vessels 

 which divide like arteries into smaller ramifi- 

 cations on the lung, and may therefore be com- 

 pared to pulmonary arteries. The capillary 

 branches of these vessels transmit the blood 

 into the commencing pulmonary veins, which, 

 uniting into larger and larger branches, ter- 

 minate in a third longitudinal vessel (o), 

 situated on the second portion of intestine. 

 This last-mentioned vessel, which may be con- 

 sidered as the great pulmonary vein, sends 

 branches on the intestine which open into the 

 wide part of the main artery, and thus the 

 blood is carried to the place whence it set out. 

 According to Delle Chiaje, the principal vein, 

 after diminishing in width, opens into the 

 oblong sac which is connected with the vessels 

 of the feet, and out of this bag six vessels 

 issue. One of these is the great artery, which 

 runs along the intestine ; the other five are the 

 vessels of the tentacula and feet previously 

 described ; each of them sends four branches 

 forwards to the tentacula, and a long one 

 backwards between the longitudinal muscles 

 to the vesicles of the feet. 



5. Nerves. Tiedemann discovered a nervous 

 system in the star-fish. He describes it (in A. 

 aurantiaca) as consisting of a delicate white 

 cord surrounding the mouth, in form of a ring 

 immediately on the outside of the circular 

 vessel into which the heart opens, and of di- 

 verging filaments which arise from the annular 

 cord opposite the rays. (Fig. 23.) There are 



Fig. 23. 



c,feet ; e,feet cut across ; /, apertures for the feet. 



three filaments for each ray; one runs along 

 the under surface in the median line, and ap- 

 pears to send small branches to the feet ; the 

 other two, which are shorter, pass between the 

 first and second segment of the ray into the 

 interior of the body, and are probably distri- 



buted to the stomach. Tiedemann could dis- 

 cover no ganglia, but others describe minute 

 ganglia as existing at the points where the 

 diverging filaments originate.* 



The Echinodermata have not generally been 

 supposed to possess any other sense than that 

 of touch. Professor Ehrenberg has how- 

 ever recently called attention to certain parts 

 in the Asterias, which he is disposed to re- 

 gard as organs of vision .f These have the ap- 

 pearance of small red spots, one of which is seen 

 at the extremity of each ray. They have been 

 long known to exist in several species of Asterias, 

 but no one ever assigned to them any particular 

 use till lately, when Professor Ehrenberg, struck 

 with their resemblance in aspect to the eyes of 

 Entomostraca and Infusoria, conjectured that 

 they might be of the same nature. He states that 

 he has traced the long nerve of the ray as far 

 as the extremity, where it swells into a sort of 

 ganglion with which the red point or supposed 

 eye is connected. 



In the Echinus Tiedemann observed fine 

 filaments on the internal surface of the 'mem- 

 brane which fills the inferior opening of the 

 shell, and on the dental apparatus and the 

 longitudinal vessels of the feet, from which he 

 inferred that a nervous system probably existed 

 in the Echinus analogous in form to that of the 

 Asterias. In the same way he was led to sus- 

 pect the existence of such a system in the 

 Holothuria, though by dissection he could make 

 out nothing more than several exceedingly 

 delicate filaments, some of which were situated 

 in the neighbourhood of the mouth, and ap- 

 peared to enter the tentacula, and others lay 

 on the longitudinal muscles. Dr. Grant de- 

 scribes a connected nervous system in the Echi- 

 nus and Holothuria, but without mentioning 

 on whose observations his description, which 

 we here transcribe, is founded. " A nervous 

 chord," he states, " is seen round the oeso- 

 phagus of the Echinus, which sends delicate 

 white filaments to the complicated muscular 

 and sensitive apparatus of the mouth ; other 

 nerves are seen extending upwards from the 

 same cesophageal ring, along the course of 

 the vessels in the interior of the abdominal 

 cavity. In the Holothuria the nervous sys- 

 tem is extensively developed. Interior to 

 the osseous apparatus of the mouth is a white 

 nervous ring around the oesophagus, from 

 which nerves pass outwards to the large 

 ramified tentacula around the mouth, and 

 others extend upwards along the course of 

 the eight strong longitudinal muscular bands. 

 Fine white filaments are likewise seen passing 

 inwards to the stomach and alimentary ap- 

 paratus.";]; In a recent notice of some obser- 

 vations on the Echinus by M. Van Beneden, it 

 is stated that he distinctly recognized a nervous 

 collar surrounding the oesophagus. 



6. Generative organs. The only organs 

 hitherto discovered in the Echinodermata, which 



* Grant's Comparative Anatomy, p. 184. 

 t Miiller's Archiv flic Anatomic, Physiologic, 

 &c, 1834. p. 577. 



$ Comparative Anatomy, p. 184. 



