MUSEUM OF COMPARATIVE ZOOLOGY. 1 1 i 



correspondence in details is wanting. The dermal glands are hardly 

 more than similar in type, and a direct correspondence between the 

 different kinds is not to be found ; for the bicellular are entirely want- 

 ing in S. Gouldii, and the multicellular of S. nudus agree with neither 

 group described fur S. Gouldii. Whether the non-glandular organs of 

 Andrews correspond to the small papillae described above cannot be 

 definitely determined, on account of the brevity of Andrews's description 

 and the laclj: of figures. On the other hand, Andrews lias emphasized 

 the fact that a close agreement exists between the dermal bodies of 

 S. Gouldii and those of various Phascolosomes. Agaiu, hi the arrange- 

 ment of the musculature, in the uniform unhanded circular layer, in 

 the absence of diagonal fibres, and in numerous other details, S. Gouldii 

 is unlike S. nudus, and in the same degree that the former resembles 

 Phascolosoma. In the light of these facts, a modification of the generic 

 characters given by Selenka ('S3) to Sipunculus, which include S. Gouldii 

 in the same genus with S. nudus, would seem advisable. 



Striking as is the similarity between the anatomy of the nervous sys- 

 tem in the Annelids and in the Sipunculids, certain characteristic dif- 

 erences are worthy of note. The peripheral system of plexuses is 

 very highly developed in the latter, and consists almost entirely of 

 fibres, whereas the dermal plexus of Capitellids, Nemertines, and Poly- 

 chsets is composed largely of ganglionic cells. In the ventral nerve 

 cord of Sipunculids there is no metameric arrangement of the lateral 

 branches, nor any concentrations of the ganglionic elements in the cord 

 itself. On the other hand, there is present a splanchnic nerve and an 

 intestinal plexus in both Sipunculids and Aunulata, and the complicated 

 structure of the supracesophageal ganglion in Sipunculus agrees in gen- 

 eral with that of various Annelids and Nemertines. 



As regards the histology of the central nervous system, it will be 

 noticed that the description given in this paper for S. nudus corresponds 

 closely with that given by Rohde ('87) for Chsetopods, and by Burger 

 ('90) for Nemertines. It is of interest, however, to note more exactly 

 the points of likeness and difference. If further investigation should lead 

 to the discovery of a minimal cell body for the nervous nuclei (Nerven- 

 kerne) of Rohde, — and I think this probable on account of the extreme 

 difficulty experienced by Burger ('90, p. 106) and myself in finding this 

 cell substance, — then these nervous nuclei would correspond in general 

 character and occurrence with the first class of ganglionic cells described 

 by Burger in Nemertines, and with the first type in Sipunculus. The 

 first class of Rohde agrees in general with the second of Biirger ; but 



von. xxi. — no. 3. 12 



