532 COMPARATIVE ANATOMY CHAP. 



In the meantime new pairs of outwardly projecting lateral lobes (rudiments of 

 tube-feet) have appeared on the hydroccel bulgings, which have developed into the 

 radial water vascular trunks ; these new growths always appear distally to those 

 already formed and proximally to the median terminal lobe (terminal tentacle). 



The nervous system develops as an epithelial circular cushion in the oral area, 

 even before the mouth has broken through its centre. 



The skeleton receives the addition of fifteen new plates on the apical side outside 

 of the basals, five being radial and five pairs interradial. 



In each interradius orally (on the thirteenth day) a plate forms between the 

 separate proximal pairs of ambulacral plates. These five plates are the rudiments 

 of the orals (odontophores). 



At the sides of the ambulacral plates the adambulacral plates appear. The 

 remaining pairs of ambulacral and adambulacral plates arise in the same order as 

 the pairs of tube-feet, always proximally to the terminals of the arms, and distally 

 to those already formed. 



The five first and the five second pairs of ambulacral plates unite with the five 

 first pairs of adambulacral plates to form the oral skeleton. 



The five radial outgrowths of the intestine quickly grow into the arms, forking, 

 and thus producing the ten brachial diverticula of the digestive sac. Five pairs of 

 small interradial outgrowths on the water vascular ring represent the rudiments of 

 Tiedemann's bodies. None of the tube-feet at first have suckers. The formation 

 of the nerve ring is followed by that of the radial nerve ridges, which, like the 

 former, are epithelial in position, persisting as such even in the adult Asteroid. The 

 continuous ciliated covering of the larva is at no time interrupted, but passes 

 direct into the ciliated covering of the Asteroid. 



We shall not enter upon the accounts given of the rise and development of the 

 blood vascular system, since there is nothing more problematical in the anatomy 

 of the adult Asteroids than this system. 



Where, among Asteroids, a typical Bipinnaria larva is developed, the meta- 

 morphosis which produces the young Asteroid seems to resemble in essentials that 

 of Asterina. The rudiment of the young Asteroid is found in the posterior part of 

 the larva which contains the swollen mid-gut. At first, as in Asterina, this 

 rudiment is double, i.e. it consists of an oral rudiment, arising close to the hydroccel, 

 and an apical rudiment, the two uniting round the intestine. The larger anterior 

 portion of the larval body, together with the ciliated rings of the Bipinnaria, are, 

 like the larval organ of Asterina, gradually resorbed. 



E. Ontogeny of the Ophiuroidea. 



According to the present state of our knowledge the development of the Ophiur- 

 oidea does not appear to differ so greatly from that of the Asteroidea, in spite of 

 the difference in shape of the larvte, as to need detailed description. We shall 

 therefore limit ourselves to a few points. 



Development of the hydro - enteroccel. The first rudiment of the hydro- 

 enterocoel has not been observed with as much certainty as could be desired. In 

 the quite young Pluteus larva an enteroccel vesicle lies at each side of the oesophagus. 

 Somewhat later the larva possesses, besides this pair of vesicles, another pair of 

 enteroccel vesicles at the sides of the stomach-intestine, these latter having, as it 

 appears, been constricted off from the former. The left anterior vesicle at this 

 stage enters into communication with the exterior through the dorsal pore (water 

 pore). On the left side there now arises, between the anterior and the posterior 

 mteroccel vesicles, apparently by constriction from the latter, a third new vesicle, 



