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. 6. Moreover, an entirely 
new band appears, also ciliated, 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.d. At the apex ofthe 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 cesophagus, @., a stomach, s., 
and a short intestine, 7, the mouth being placed at m., 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 cesophagus, 
probably, however, not connected with it, even at this early stage. 
Its cavity communicates with the exterior by a tube, c. 4, the pore 
of which is on the dorsal side of the larva slightly to the left of the 
median line, d. f. A thread-like muscle band passes down from the 
apical plate to the sac mz. 
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, p. 4., the meso- 
blastic pouches, m. p. [Fig. 5], the heart, #., and three pairs of gills, 2. =. 
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 ecaie s 
while nets have called it an —— gill. 
