118 BULLETIN OF THE 



opment of Ophiothrix, " Ces deux masses cellulaires ne sont pas, comma 

 I'a suppose M. Balfour, dues k des diverticulum de I'archenteron, sem- 

 blables aux cavites vaso-periton^ales des Holothurins, mais elles sout 

 des produits d'une formation directe, comme cela a lieu pour I'estomac." 



PI. I. fig. 18, shows a larva slightly older than the gastrula last 

 described. If we look at this larva from the flat or ventral side, we 

 notice on each side of a single opening * small pigmented protuberances. 

 These prominences in the future growth of the larva become more and 

 more extended, and even in their earliest form give evidence that they 

 develop into the lateral arms of the pluteus. The larva is now three 

 days old, and has begun to assume a form like the youngest Ophiuran 

 pluteus described by others. The longitudinal axis is .18 mm. ; the dis- 

 tance from the tip of one lateral prominence to the opposite is .16 mm. 



Tne anal lobe has grown more pointed than in the larva just described, 

 the body of which is about spherical. The oral lobe is smaller than the 

 anal, although similar ro it in form. It is as yet undivided. A mouth 

 leading into a cavity opens on the upper pole on the ventral side of the 

 oral lobe, and a broad band of cilia extending along the lateral arms 

 surround the mouth, the oral lobe, and the ventral region of the body. 

 The opening thus surrounded by a ciliated band is easily seen. Its lips 

 are richly ciliated. 



The limestone rods have already been formed in the body, PI. I. 

 fig. 19. There are two centres of formation of these bodies; but these 

 centres of calcification are at first not joined. The limestone rods, sp, 

 originate as spicules with three prongs. One prong extends into the 

 lateral rod, another in the direction of the anal lobe, and a third into 

 the oral lobe. Later, a fourth process is formed from the common 

 union of the three already mentioned, which extends to the middle line 

 of the dorsal side. 



The amoeboid or mesoblastic cells are formed throughout the region 

 of the larva, between the epiblast and hypoblast. They are spherical, 

 sometimes branched, forming suspensoria connecting the wall of the 



* Still further observation is necessary to show whether the mouth of the gas- 

 trula of Ophiurans becomes the mouth of the pluteus, or whether, as in Holothu- 

 rians (Cucumaria,) Selenka, the Starfishes (Asterias vulgaris,) A. Agassiz, (Aster- 

 ina,) Ludwig, and in some Echinoids (Strongylocentrotus) according to Krohn and 

 A. Agassiz, the gastrula mouth becomes a vent. I have not observed an anus in tlie 

 pluteus of Ophiopholis. The single opening is, therefore, supposed to be the gas- 

 trula mouth. Whether, as in some other Echinoderms, a new opening is formed, 

 and the gastrula mouth becomes an anus or not in later stages, was not observed. 



