540 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



post-oral. Of the latter, one, at the posterior end, is circular, 

 while the other is inclined obliquely to the axis of the body, 

 so that anteriorly and superiorly it reaches the anterior ex- 

 tremity, while posteriorly it occupies nearly the middle of the 

 body. On the ventral face a deep groove separates it from 

 the prse-oral ciliated band, and in this groove the mouth is 

 situated. The margins of the prse-oral and post-oral ciliated 

 bands are deeply sinuated, and they come into contact in the 

 median dorsal line. A wide gullet leads from the mouth, and 

 opens into the gastro-intestinal portion of the alimentary 

 canal, which passes backward in the middle line to terminate 

 in the anus, at the hinder end of the body. About the middle 

 of the dorsal face of the body there is a circular pore (Fig. 

 153, I. d), whence a canal leads to a rounded sac which lies 

 on the junction between the gullet and the stomach. The 

 sac gives off two lateral short diverticula, which embrace the 

 oesophagus. A delicate band, apparently of a muscular na- 

 ture, connects the summit of the w T ater-sac with that part of 

 the dorsal aspect of the body at which the prae-oral and post- 

 oral ciliated bands unite. Here two eye-spots are developed. 

 A constriction separates a rounded gastric from a tubular in- 

 testinal division of the alimentary canal. Diverticula of the 

 gastro-intestinal part of the alimentary canal give rise to two 

 pairs of discoidal bodies, from which, apparently, the meso- 

 blast and the perivisceral cavity of the JBalanoglossus are de- 

 veloped. 



From the sides of the oesophagus a series of diverticula 

 are given off, which unite with the ectoderm, open external^, 

 and become the gill-pouches. When only two of these bran- 

 chial apertures are formed, they are said by Metschnikoff to 

 have a striking resemblance to those of Appendicular 'ia. A 

 pulsating vesicle — the so-called " heart " — makes its appear- 

 ance close to the water-sac. The anterior end of the body, 

 in front of the mouth, now elongates, and is converted into 

 the proboscis ; while the post-oral region loses its ciliated 

 bands, and, lengthening, becomes the long body of the adult 

 worm. 1 (Fig. 153, II., III.) 



The Cii^etognatha. — The genus Sagitta? which is the 



1 See Agassiz, " The History of Balanoglosfiiis and Tornarla " (" Memoirs 

 of the American Academy of Arts and Sciences," 1873) ; and Metschnikoff, 

 " Untersuehungen fiber die Metamorphose einiger Seethiere." (Zcitschrift 

 fur taiss. Zoologie, xx..lS7 A ). 



2 See Busk, Quarterly Journal of Microscopical Science, 1856. Leuckart and 

 Pagenstecher, " Archiv iiir Anatomie," 1858. 



