538 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



complete division, and the embryo leaves the egg as an oval 

 morula, covered with vibratile cilia. In the next stage ob- 

 served, the embryo is cylindroidal, and is provided with a 

 mouth at one end and an anus at the other. The commence- 

 ment of the straight and simple alimentary canal has the 

 form of a muscular bulb or proboscis. There are two pairs 

 of rudimentary appendages, each containing two seta?. The 

 number of the setigerous appendages increases up to five 

 pairs, and the intestine begins to show indications of diver- 

 ticula ; but, in the latest stage observed, the cirri had not 

 made their appearance, and the body was still comparatively 

 narrow. 



Metschnikofr" regards Myzostomum as a parasitic form of 

 a pohychsetous Annelid ; and there is much to be said in 

 favor of this suggestion ; though, in some respects, it rather 

 approaches the Hirudinea. 



The presence of cilia on the surface of the body and of 

 protractile seta? in the parapodia excludes 31yzostomum 

 from the Arthropoda y while Metschnikoff has justly com- 

 pared its larval state with that of Syllis. Sufficient doubt, 

 however, still adheres to the determination of the true place 

 of Myzostomum, to lead me to discuss it apart from the An- 

 nelids. 



The Enteropneusta. — The very singular animal JBalano- 

 glossus, which is the only known example of this group, is 

 an elongated, apodal, soft-bodied worm, with the mouth at 

 one end of the body and the anus at the other (Fig. 153, 

 III.). The mouth is surrounded by a sort of collar, or promi- 

 nent lip, within the margin of which springs a long probos- 

 cidiform median appendage, which is hollow within and has a 

 terminal pore. On the same side as that from which the pro- 

 boscis springs, the anterior region of the body presents an 

 elongated, somewhat flattened area, bounded by raised longi- 

 tudinal folds. On each side of this area is a longitudinal se- 

 ries of apertures — the branchial apertures. The latter com- 

 municate with saccular dilatations of the anterior part of the 

 alimentary canal, and these branchial sacs are supported by 

 a peculiar skeleton. 



No nervous system, nor any organs of sense, have yet been 

 certainly made out. 



According to Kowalewsky, 1 who was the first to elucidate 



1 " Anatomie des Balanoglossus." (" Mem. de l'Acad. Imp. de St.-Peters- 

 bourg," 1866.) 



