ENTEROPNE US TA. 485 



ending blindly at their two extremities. The two spurs of the water- 

 vascular vesicle, which in the Tornaria stage rested upon the stomach, 

 now grow round the oesophagus, and form an anterior vascular ring, 

 which Agassiz describes as becoming connected with the heart, though it 

 still communicates with the exterior by the dorsal pore and seems to be- 

 come connected with the remainder of the vascular system. According 

 to Spengel (No. 572) the dorsal vessel becomes connected with the heart, 

 which remains through life in the proboscis : the cavity of the water- 

 vascular vesicle forms the cavity of the proboscis in the adult, and its 

 pore remains as a dorsal (not, as usually stated, ventral) pore leading to 

 the exterior. 



The eye-spots disappear. 



Tornaria is a very interesting larval form, since it is intermediate 

 in structure between the larva of an Echinoderm and trochosphere 

 type common to the Mollusca, Cruttopoda, etc. The shape of the 

 body especially the form of the ventral depression, the character of 

 the longitudinal ciliated band, the structure and derivation of the 

 water-vascular vesicle, and the formation of the walls of the body 

 cavity as gastric diverticula, are all characters which point to a 

 connection with Echiuoderm larvae. 



On the other hand the eye-spots at the end of the prse-oral lobe *, 

 the contractile band passing from the oesophagus to the eye-spots 

 (fig. 273), the two posterior bands of cilia, and the terminal anus are 

 all trochosphere characters. 



The persistence of the prse-oral lobe as the proboscis is interesting, 

 as tending to shew that Balanoglossus is the surviving representative 

 of a primitive group. 



BIBLIOGRAPHY. 



(567) A. Agassiz. "Tornaria." Ann. Lyceum Nat. Hist. vni. New York, 1866. 



(568) A. Agassiz. " The History of Balauoglossus and Tornaria." Mem. Amer. 

 Acad. of Arts and Scien., Vol. ix. 1873. 



(569) A. Gotte. "Entwickluugsgeschichte d. Comatula Mediterranea." Archie 

 fiir mikr. Anat., Bd. xn., 1876, p. 641. 



(570) E. Metschnikoff. " Uutersuchungen iib d. Metamorphose, etc. (Tor- 

 naria)." Zeit. fiir wiss. Zool., Bd. xx. 1870. 



(571) J. Miiller. "TJeb. d. Larvenu. Metainor. d. Echinodemien." Berlin Akad., 

 1849 and 1850. 



(572) J. W. Spengel. " Bau u. Entwicklung von Balanoglossus." TageU. d. 

 Natitrf. Vers. MiincJien, 1877. 



1 It would be interesting to have further information about the fate of the thicken- 

 ing of epiblast in the vicinity of the eye-spots. The thickening should by rights be the 

 supra-cesophageal ganglion, and it does not seem absolutely impossible that it may give 

 rise to the dorso-median cord in the region of the collar, which constitutes, according 

 to Spengel, the main ganglion of the adult. 



