MYZOSTOMIDA FROM THE RED SEA. 95 



I think there can be little doubt but that v. Stummer was right in 

 considering these cells to be unicellular glands. In M. costcdum 

 these glands may have a digestive function ; they certainly form 

 the only glandular tissue in connection with the alimentary canal 

 in this species. 



Nervous System. 



The best and most complete account of the Myzostomid nervous 

 system is that published by I^ansen in 1885 (15) ; it was based 

 on investigations made on several species, especially M. giganteum 

 Nansen, M. graffl Nansen, and M. cirriferum Leuckart. In these 

 forms, which can be regarded as quite typical, the nervous system 

 consists of a feebly-developed circumpharyngeal ring communi- 

 cating with a ventrally-situated unsegmented nerve-mass, from 

 which eleven pairs of nerves arise. Of these nerves five on each 

 side are large and supply the five parapodia, as well as the parts 

 of the body-margin on both sides of them, whilst the other six 

 are more slender and alternate with the larger nerves supplying 

 the parts of the body between the parapodia. 



Text-fig. 20. 



r- n: 



-/Vr' 



— n: 



n? 



Diagram to show the origin of the paired nerves from the ventral nerve-mass in 



Myzostoma costatum. 



N'-N*. The five pairs of large nerves, n^-n^. The six pairs of smaller nerves. 



C. Circumpharyngeal commissures. 



Nansen did not state the nerve-supply of the " suckers," but 

 V. Stummer (16) believes these organs to be innervated by branches 



