THE DEVELOPMENT OF BALAXOGLOSSUS. 



539 



the true nature of Balanoglossus, the vascular system con- 

 sists of a dorsal and a ventral vessel. At the posterior end 

 of the branchial region the former divides into a superior and 

 an inferior dorsal, and two lateral, trunks. The superior 

 trunk passes forward, and, at the anterior end of the body, 

 divides into two descending branches, which unite with the 

 ventral trunk. The inferior dorsal trunk supplies the bran- 

 chiae, of which the lateral trunks are the efferent vessels. 



For the pharyngeal branchiae of Balanoglossus, the only 

 parallels to be found are among the Tunicata and the Verie- 

 brata. On the other hand, the larval form of this anomalous 

 creature is generally Annelidan or Turbellarian, with very 

 close and special resemblances to the Echinopaedia of some 

 Ech inodermata. 



The young of Balanoglo&sus was first observed by Miiller, 

 who called it Tornaria, and regarded it (as did all succeeding 

 observers until its true nature was discovered) as an Echino- 

 derm-larva, on account of its extraordinary resemblance to 

 the larvae of Star-fishes (Fig. 153, I.). 



1 9"VS0- ~&ir\i 



Fig. 153.— Balanoglossus. (After A. Agassiz.) 



I. The Torn-aria larva, side-view (about T V of an inch Ion?): a. anus; 6, vessels 

 leading to the dorsal pore (d) from w. the sac of the water-vascular system ; w\ 

 prolongation of the sac; A, heart; I, intestine; s, stomach; 0, oesophagus; m, 

 mouth; v, u\ lobes of the alimentary canal; mb, muscular band running from 

 the eye-speck (e) to the water-vascular sac. 



II. A young Balanoglossus— letters as before, except g, the first formed branchial 

 stigmata. 



III. A more advanced Balanog ossus ; e, the collar; p, the proboscis. 



It is an elongated ovoid body, provided with three bands 

 of cilia, one of which is prae-oral, while the other two are 



