72 BALANOGLOSSIDA. 



or invagination (according to the species) of a dorsal median 

 tract of collar ectoderm. This collar nerve-cord, which is called 

 the medullary cord, is always continuous with the outer ecto- 

 derm at the front and hind end of the collar (and in some species 

 by the dorsal roots, see below). At these points (Fig. 60) there 

 is almost always a canal the medullary canal which in some 

 species is continued through the whole length of the cord. The 

 central canal, whether it is confined to the anterior and pos- 

 terior ends or extends throughout the whole length of the cord, 

 opens to the exterior by the anterior and posterior nerve pores, 

 the former being placed at the base of the proboscis stalk within 

 the anterior projecting rim of the collar, and the latter at the 

 bottom of the groove (atrial) which is found at the junction of 

 the collar and trunk (Fig. 60). 



The fibrous matter of the trunk dorsal nerve-cord is continued along the 

 ventral side of the medullary cord. In the genus Glossobalanus, however, 

 the fibrous matter is continued right round the medullary cord, being found 

 on its dorsal surface also (Figs. 58, 59). 



In those cases in which the central canal is not present as a continuous 

 tube, it appears to be represented by a number of isolated cavities which 

 are frequently surrounded by elongated radiating cells. * In Schizocardium 

 peruvianum and Dolichoglossus merechkovskyi the medullary cord is quite 

 solid, there being neither neuropores nor isolated cavities. 



There is frequently present at the anterior and posterior ends of the 

 medullary cord an ectodermal pit, with the ectodermal lining of which the 

 nerve cord is continuous. The anterior and posterior neuropores when 

 present open into these pits, or if the cord is entirely solid, as in the two 

 species just named, the medullary cord is continuous with the ectoderm 

 lining them. It is difficult to say whether the pits should be regarded as 

 terminal parts of the central canal or as independent structures. Some 

 importance has been attached to them by Spengel, but as in many cases 

 it is exceedingly difficult to distinguish them from the adjacent parts of the 

 medullary canal, it is perhaps better to regard them as the terminal and, 

 if the central canal is elsewhere obliterated, as persistent portions of this 

 canal. The dorsal nerve cord has the same relation to these pits as it has 

 to the cord, i.e. it lies in their ventral walls. They are sometimes dilated 

 and the anterior pit may project backwards so as to overlap the dorsal 

 side of the adjacent part of the medullary cord (Spengelia alba, Schizo- 

 cardium), thus recalling in some degree a cerebral vesicle of the vertebrate 

 brain. 



The condition of the central canal varies much even in different species 

 of the same genus. It is present as a complete canal, traversing the whole 

 length of the medullary cord and opening at each end by a neuropore, only 

 in the genus Ptychodera (Pt. flava, erythrea, bahamensis), in Glossobalanus 



* In some cases these isolated cavities are said to coexist with a central 

 canal without however communicating with this latter (e.g. Sch. brasil- 

 iense in some individuals). 



