200 ECHINODEEMATA. 



and as they present a large surface to the action of the water and 

 receive numerous vessels from the circular trunk that surrounds the 

 mouth, they may, no doubt, very well contribute to the complete ex- 

 posure of the blood to the influence of the surrounding medium. 



(524.) Little is known concerning the nervous system of the Echini : 

 a few delicate filaments have been observed in the neighbourhood of the 

 oesophagus, apparently of a nervous character, communicating with a 

 nervous ring placed in that vicinity, resembling that already described 

 in Asterias. 



(525.) The Echini, like the Star-fishes, are bisexual, and in the 

 structure of their reproductive organs display, if possible, greater sim- 

 plicity than even the Asteridae above described. The ovaria are five 

 delicate membranous bags, quite distinct from each other, that open 

 externally by as many delicate tubes, or oviducts, as we may term them. 

 The apertures through which the eggs escape are easily seen upon the 

 outer surface of the shell, placed around the anus, and are recognizable 

 not merely by their size, but from the circumstance of each perforation 

 being placed in the middle of a distinct oval plate of the shell, distin- 

 guished by zoological writers as the ovarian pieces. The membranous 

 sacs in which the ova are secreted vary in size in proportion to the 

 maturity of the eggs contained within them, and at certain times of the 

 year are enormously distended : it is in this state that the " roe of the 

 Sea-egg," as the ovaria are commonly called, is used as an article of 

 food ; and in some countries, especially upon the shores of the Medi- 

 terranean, they are eagerly sought after, when in season, by divers 

 employed to procure them. The corresponding organs in the male sex 

 are only distinguishable by the spermatozoa contained in their interior 

 instead of ova. 



(526.) At the earliest period observed by Miiller, the larval Echinus 

 (fig. 99, 1) had the appearance of a transparent dome-like disk, 

 hollowed out inferiorly, and having its margin prolonged into long, 

 slender, diverging processes supported on calcareous pieces deposited in 

 their substance, and giving the whole animal somewhat the appearance 

 of a timepiece standing on many legs (A, B, p, E), four of which (F, E) 

 constitute a sort of framework surrounding the oral apparatus. 



(527.) The arrangement of the locomotive apparatus of these larva? 

 is very peculiar, consisting of four epaulet-like wreaths of long cilia 

 situated upon the dome-shaped body of the animal, and of numerous 

 ciliated fringes spread over the arms and in the vicinity of the oral 

 organs. 



(528.) The mouth is a triangular orifice (fig. 99, 1, a) furnished with 

 broad lips, and leads immediately into the stomach (d), which is a cul- 

 de-sac, situated in the interior of the body. 



(529.) In this condition the larvae are not more than half a line in 

 length, and move freely about in the water, rowed along by the action 



