THE II1ST0RY OF BALANOGLOSSUS AND TORNARIA. 427 



spots are closely crowded along the broad belt of anal vibratile cilia, as well as 

 smaller spots on the longitudinal bands of smaller cilia. The eye-specks .ire black 

 and extremely prominent. The large and powerful cilia of the broad anal belt move 

 comparatively slowly, more like the cilia of the embryos of Molluscs, as has already 

 been observed by Muller. 



Metschnikoff, in the oldest stage of Tornaria observed by him, speaks of two 



round bag-shaped diverticula of the oesophagus, having an internal structure similar 

 to it, which with advancing age only increase in size. In our Tornaria the mode 

 of formation of the pouches has been traced, and the gills (I will call them at once 

 by their true name, from their function in the adult Balanoglossus) have been traced 

 from their first appearance, a single .pair, till there were four such pairs in the Tor- 

 naria; this is previous to any other changes. The gills are at first simply slight folds, 

 one on each side of the dorsal part of the oesophagus: .the folds gradually change 

 into loops, then we have a series of folds, and subsequently a series of four loops, 

 and finally, when seen from the dorsal side, the loops appear closed, forming a set 

 of four funnels on each side, opening into the oesophagus, which from above look 

 like round rings adjoining one another. These changes are readily followed in the 

 figures illustrating this part of the development. 



The passage of Tornaria into young Balanoglossus is very sudden, taking place in 



f 



a few hours ; but, unlike the transition from the Pluteus into the Echinoderm, there 

 is no resorbition of any portion of the larva. The whole transformation consists 

 simply in a lateral contraction of certain parts and an elongation of others, but 

 which is sufficient, with the disappearance of the longitudinal bands of cilia, to alter 

 completely the general aspect of the larva. The first trace of the change is the 

 opaqueness of the larva: it loses its transparency, and somewhat its activity; the 

 whole of the stomach is drawn down towards the intestine ; the oesophagus becomes 

 reatly elongated ; and the transverse oral vibratile band is now above the junction 

 of the stomach with the oesophagus. The oesophagus is lengthened so much that 

 the water-system no longer rides upon the anterior part of the stomach, but is 

 placed immediately above the opening of the . mouth. The intestine has become 

 depressed into a triangular pouch, and the stomach forms a few folds to accommo- 

 date itself to its present circumscribed space : these folds are the first trace of con- 

 volutions similar to those of the adult. During this process of lengthening of the 

 posterior part of Tornaria, the anterior part is also greatly elongated, assuming 

 somewhat the shape of an elliptical proboscis. In front of the circular band of cilia 

 are plainly seen the four pairs of gills on each side of the oesophagus : this is now 



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