C20 SUMMARY OF CURRENT RESEARCHES RELATING TO 



graM are now species, both taken from Antcdon celtica. M. giganteum 

 is very like ]\I. gigas, but has a more robust and less flattened body ; 

 M. grajji is like M. marginatum, but is distinguished by its " tongue- 

 indented margin," and by the twenty cirri, one on each of the twenty 

 tongues of the margin. The nervous system is altogether on the 

 annelid and arthropod type, appears to be greatly differentiated, and 

 to lio at some distance from the surface, the ventral cord being 

 separated by a thick muscular layer from the ectoderm. The cerebral 

 ganglion has no special sheath, but the circular commissures have a 

 double neurilemma-sheath. The proboscidial nervous system is well 

 developed, three pairs of nerves arising from the oesophageal ring 

 and connecting it with the tentacular nerve-ring in the proboscis. 

 The ventral cord is oblong, short, and concentrated, has a double 

 nerve-sheath, and an intermediate nerve passing between the two 

 longitudinal commissures ; this communicates with the commissures 

 by alternating branches. Eleven pairs of nerves issue from the ven- 

 tral cord, five of which are larger than the other six : to Mr. Nausen 

 the segmentation of the cord is not as obvious as it was to Mr. 

 Beard. 



There is some difficulty in determining whether the so-called 

 parapodial ganglia are really nervous, and not glandular in nature. 

 In M. giganteum they were found to consist of two ganglia con- 

 taining a large number of cells; these are multipolar, and each sends 

 a prolongation towards the external extremity of the parapodium. In 

 31. graffi the two ganglia of each parapodium are united, and contain 

 a few (six to seven) gigantic cells. At the inferior extremity of the 

 ganglion there is a peculiar organ which has the form of a glass bulb- 

 receiver with the long receiver-neck passing towards the extremity of 

 the parapodium ; the globular portion consists of several concentric 

 layers, within which there is a substance which appears to be 

 coagulated. 



The outer neurilemma-sheath consists of a stout homogeneous 

 membrane which stains deeply ; no nuclei were found in it, but many 

 were seen adhering to its outer side ; it is probably a cuticle, and is 

 derived from the adjacent layers of connective tissue ; it gives off 

 septa which pass into the inner neurilemma by the inner parts of the 

 ventral nerve-cord. The inner neurilemma-sheath is formed of re- 

 ticulating layers of connective tissue, which form membranes for the 

 fibrillar cords and the ganglionic cells. These last are mostly 

 unipolar ; the prolongations from them either pass directly into a 

 peripheral nerve to form a nerve-tube, corresponding to the cylinder- 

 axis of vertebrates, or they become broken up into the fibrillar 

 reticulation of the central mass, from which nerve-tubes arise. In 

 section, the protoplasm of the ganglion-cells always appear to be 

 spongy. Direct division of the nuclei has been often observed in the 

 ganglionic cells, but in no case karyokinesis. The author doubts the 

 accuracy of Beard's account of the development of the nervous system, 

 and thinks that it is of the type common to Annelids. 



There does not appear to be a definite body-cavity, but rudiments 

 of it are apparently to be found in the cavities in which the ova arc 



