1901.] NATURAL SCIENCES OF PHILADELPHIA. 683 



of the ventral braiu lobes, about .00 mm. behind the ventral com- 

 missure. Both cells lie in the same transverse plane. In one 

 very favorable specimen the nerve tubule of one coll could be 

 traced into the fibrous core of the ventral lobe. These cells are 

 elongated, with the greatest diameter at the rounded proximal 

 end, and surrounded by a sheath of connective tissue fibres, 

 Plate XL, fig. 3, Cn. T. S. The length of the cell body is about 

 .058 mm., the greatest width about .029 mm., the diameter of the 

 nucleus .012 mm. The cytophism stains a pale violet, htematoxy- 

 lin and eosin stain, and has a slightly granular appearance. The 

 somewhat oval nucleus is proximally placed, the chromatin is dis- 

 tributed around the periphery, and one large nucleolus is present. 



From the position, size and structure of these cells it seems not 

 unfitting to term them neurochord cells. 



Biirger has observed that neurochord cells occur in those forms 

 that swim freely, and he thinks there may be some correlation 

 between the occurrence of neurochords and the swimming habit. 

 While it would be hasty to say that Zjgeupolia has not the power 

 of free swimming, it has not been observed swimming, either in 

 nature or in captivity. In its native habitat it is always found 

 below the surface of the sand, and while in an aquarium it never 

 rises to the surface, but remains on the bottom, burrowing in the 

 sand, if there is any present, and surrounds itself as soon as possi- 

 ble with a slime sheath to which particles of sand adhere. When 

 placed in a shallow dish of water the head is usually kept erect 

 and continually swaying about, but there is little or no movement 

 of the body as a whole. 



It should be mentioned here that one pair of large ganglion 

 cells, from their structure and position evidently neurochord cells, 

 has lately been observed by the writer in Micrura caxa. It is not 

 improbable that the occurrence of these cells among the Nemer- 

 teans is more common than is generally supposed. 



The outer neurilemma is but slightly developed in Zijgeupolia . 

 It is found around the ventral lobes, but does not occur to any 

 extent around the dorsal lobes or along the lateral chords. 



3. Sense Organs. — a. The Cerebral Organs. — The cerebral 

 organs appear in life, Plate XL, figs. 1, 16, ('. Org., as rather pear- 

 shaped bodies situated at the posterior ends of the dorsal brain lobes, 

 above the ventral lobes. They are silvery gray, except the poste- 



