ON THE AXATO.MY OF HISTRJOBDELLA HOMARI. 327 



slight, being shown only by the ciliated rings. Internally it 

 is better marked by the dissepiments and the nephridia. The 

 two halves of the cord remain separate throughout their 

 course, uniting at their ends in a small ganglion. Such a 

 nervous system can hardly be compared with that of His- 

 triobdella. 



According to Pierantoni (26), the nerve-cells in Proto- 

 drilus are equally as difhcult to distinguish from the sur- 

 rounding tissues as in Histriobdella. While retainin": 

 their primitive position in the ectoderm, they send fibrils to 

 the tentacles and the digestive system. In the ventral cord 

 there would seem to be no localisation Avhatever of the nerve- 

 cells corresponding to the segmentation. 



8. Sense-Okgans. 



Among the sense-organs of Histriobdella are to be 

 classed the five tentacles of the head and the palps of the 

 posterior legs. All these receive nerve-fibres from the central 

 nervous system, and are armed with short, stiff, sensory hairs. 

 The most essential of the tentacles appears to be the median 

 one of the head. In the larva this is the first to appear, and 

 its nerve supply in the adult would seem to be greater than 

 that of the others. In addition to the tentacles, scattered 

 over the cuticle of the bodv are a number of cells of a sensorv 

 nature that stain readily with methylene blue. 



On the dorsal latei-al parts of the head are the sensory pits 

 described by Foettinger. These, as already mentioned, are 

 very small, and placed a short distance in front of the anterior 

 feet. Foettinger has sought to compare them with the ciliated 

 grooves of Archiannelids. They measure about 14 /.t in their 

 longest diameter, and are oval in outline. They are therefore 

 much smaller than the long grooves of Protodrilus and 

 Polygordius. In the bottom of the pit are placed a few 

 fine sensory hairs. As described by Foettinger, the anterior 

 edge of the pit is developed into a slight lip or ridge that is 

 capable of being folded completely over the pit and of 



