428 



MEMOIRS OF THE AMERICAN ACADEMY. 



divided into two portions, — one the mouth-opening of the oesophagus, the other the 

 convoluted gill-bearing part. The pigment-cells of the longitudinal undulating 

 bands of vibratile cilia, as well as the bands themselves, have almost disappeared. 

 The eje-specks alone are still extremely prominent ; the muscular band attached to the 

 anterior part of the water-system has disappeared, but powerful longitudinal muscular 

 bands as well as less marked transverse bands appear on the proboscis. The walls 

 of the water-system fiave become contracted; the water-system occupies a com- 

 paratively much smaller space in the proboscis of the young Balanoglossus than in 

 the anterior part of Tornaria. The heart can now no longer be distinguished through 

 the opj.ime walls of the contracted water-system. The opening of the dorsal pore 

 is plainly seen near the base of the proboscis. 



In somewhat older stages all trace of the undulating longitudinal bands of vibratile 

 cilia have disappeared ; minute cilia, no longer arranged in bands, cover uniformly 

 the walls of the body and proboscis. The young Balanoglossus can now be separated 

 into three well-marked regions, — the proboscis, the collar, and the short, triangular 

 abdominal portion. The elongation of all the parts of the Tornaria behind the 

 proboscis is quite marked, and the distance between the collar and the anal 

 vibratile band has become considerable. The proboscis grows more and more 

 elongated, the body comparatively narrow and slender, while the collar-s _ 

 is better distinguished from the segment above the anal vibratile band. The e 

 of the oesophagus from which the gills have developed lengthens more rapidly 

 during the last stages than any other part of the young Balanoglossus. The mouth 

 is already, as in the adult, a broad circular opening immediately under the base of 

 the proboscis, leading into an open cavity becoming strictly an oesophagus only at 

 the point where the gills have commenced to develop. The walls of the little worm 

 become more and more opaque with advancing age : near the collar they generate 

 already in the youngest stages a moderate quantity of transparent mucus, which 

 is so abundantly and rapidly generated by the adult Balanoglossus. The few irregu- 

 larly scattered pigment-cells still found upon the proboscis are the remnants 

 undulating vibratile bands. In the broad anal band the cilia have lost much of 

 their energy, vibrating but feebly and - extremely slowly. The little worm no longer 

 swims freely about, as it does in the earliest stage of its Balanoglossus existence 

 (slightly older than the pelagic one caught by MetschnikofF), but creeps rapidly over 

 the bottom by means of its proboscis, which acts as a sort of propeller, taking 

 m water at the minute opening of the anterior extremity of the proboscis, and 

 expelling it through an opening on its ventral side immediately in front of the 



gment 



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