196 Balanoglossus. [zoe 



The four all unite at the apex of the anterior end of the larva, 

 a. p.; and since the two short ones are continuous at their other 

 ends by a cross-band in front of the mouth as are also the two 

 longer ones by a similar band behind the mouth, the whole 

 four form in reality a single band at this stage. At a lit- 

 tle older stage these bands become much more complicated by 

 being separated at the apex, and by taking on several loops in their 

 course. The details of this need not be entered into, but a general 

 idea of it can be gathered from Fig. 6, c. b. Moreover, an entirely 

 new band appears, also cihated, the cilia here being considerably 

 longer than those of the other bands. This one passes around the 

 anal end of the larva in the form of a girdle, and this form it never 

 changes as long as it exists, viz. : throughout the larval life, Figs. 

 5 and 6, c. c. b. At the apex of the larva, at the point to which the longi- 

 tudinal bands converge, is found a thickened spot in the ectoderm, 

 supposed to be nervous; and in the center of this is a pair of pig- 

 mented eye-spots, a. p. and e. s. of the figures. 



At the stage represented by Fig. 7, the only internal organs are 

 the digestive tube consisting of an oesophagus, ce., a stomach, s., 

 and a short intestine, /., the mouth being placed at ;«., and the anus 

 at a, at the posterior end of the body; and the very small beginning 

 of the "water vascular system," as it was originally called from 

 its supposed identity with that organ in the Echinoderm larva. This 

 is a single sac placed on the dorsal surface of the oesophagus, 

 probably, however, not connected with it, even at this early stage. 

 Its cavity communicates with the exterior by a tube, c. /., the pore 

 of which is on the dorsal side of the larva slightly to the left of the 

 median line, d. p. A thread-like muscle band passes down from the 

 apical plate to the sac 7mi. 



Without attempting to follow the steps of development, we may 

 pass to the condition that is presented by a larva just previous to 

 its transformation into the Balanoglossus. Such a stage is shown 

 by Fig. 4. The new organs that have appeared in addition to those 

 already described are the so-called proboscis glands,/), b., the meso- 

 blastic pouches, in. p. [Fig. 5], the heart, h. , and three pairs of gills, ^. 

 The exact origin of the proboscis gland — or vesicle as it is sometimes 

 called — is not known, neither is its function known, though in the 

 adult animal it is thought by some to be an excretory organ, 

 while others have called it an accessory gill. 



