120 ZOOLOGY. 



It consists of a narrow oesophagus (Fig. 83, ce], more or 

 less pentagonal near the mouth, dilating into the stomach ; 

 and of a terminal intestine. The long stomach passes from 

 left to right around the interior of the body, then turns up 

 toward the aboral end, and curves back in the opposite 

 course, again passing around the body from right to left, 

 forming two series of loops partly enclosing the ovaries ; it 

 is held in place by a broad, thin membrane or " mesentery." 

 The reproductive and other organs are much as described 

 in the star-fish, there being five ovaries or spermari.es, the 

 sexes being distinct. The nervous ring around the mouth 

 sends off five nerves along the ambulacra, which are accom- 

 panied by a water-vascular canal sending branches to the 

 tentacles, and a pseudo-hasmal canal, there being an oral and 

 aboral (anal) haemal ring (their presence is denied by Hoff- 

 mann), as well as an oral water-vascular ring, with five Polian 

 vesicles (present only in the true Echini and Clypeastroids), 

 a stone-canal and a fusiform tube or " heart "* next to it, 

 while the alimentary canal is accompanied by two ha?mal 

 vessels, one on the " dorsal " and the other on the free or 

 ventral side, communicating with a lacunar network in its 

 walls. 



In Echinus it is difficult to perceive any bilateral sym- 

 metry, the parts radiating, as in tbe star-fish, from the cen- 

 tre ; but in the Spatangus and allied forms it is easy to di- 

 vide the animal into a right and left side, and the body is 

 more or less elongated, as in Pourtalesia (Fig. 87), the mouth 

 being situated at one end and the anus at the other. 



The mode of development of the common sea-urchin 

 {Fig. 78) has been discovered by Mr. A. Agassiz. The earli- 

 est stages are much as described in the star-fish. The form 

 of the pluteus larva is quite remarkable, there being eight 

 very long slender arms supported by slender calcareous rods 

 projecting from the body, and. during the movements of 

 the animal, opening and shutting like the rods of an um- 

 brella. The body is provided with a sinuous row of vibra- 



* It should be observed that the latest and best observers are at vari- 

 ance regarding the structure and function of the so-called Echinoderm 

 "beart." 



