292 Memoirs of the Indian Museum. [Voi,. I, 



The question then arises can two organs placed in such antagonistic morphologi- 

 cal situations be homologous or not ? We shall content ourselves with stating the 

 problem. 



2. The nervous system, consisting of cerebral ganglia and two lateral nerve 

 cords, consists of two bilaterally symmetrical halves. These two halves are joined 

 together {i) in the head dorsally by the large commissures of the cerebral ganglia ; 

 (m) behind the anus again dorsally by a broad important commissure rich in nerve 

 cells ; {Hi) in the head ventrally by a fine commissure resembling a single nerve desti- 

 tute of nerve cells. 



Proceeding to make the condition diagrammatic, we get a picture of an animal 

 with mouth and anus surrounded by a nervous loop lying dorsal to the alimentary 

 canal. Now if we assume that the blastopore of the gastrula is the origin of both 

 mouth and anus and that the primitive nervous system of the gastrula consists of a 

 ring around the blastopore, we could easily derive Investigator from such a gastrula, 

 and the only difference between our subject and an Invertebrate with a ventral nerve 

 cord would be that the sides of the nerve ring had not, in Investigator, fused beneath 

 the alimentary canal, which is formed by the coalition of the sides of the elongated 

 blastopore. 



The most important lesson to be derived from the anatomy of Investigator appears 

 to be this, that in two closely allied groups such as the Priapuloidea and Investiga- 

 toroidea a marked difference can exist in the anatomy of the peripheral nervous sys- 

 tem (regarding the dorsal cerebral ganglia as ' central ' and all else as ' peripheral '), 

 and that changes in the position of the main longitudinal nerve trunks in cœlomates 

 may not be so difficult of accomplishment as has been thought hitherto. 



UST OF SOME PAPERS BEARING ON THE SUBJECT. 



1. Andrews, E. A. .. ,. "Reproductive Organs of Phascolo- 



soma Gouldii," Zool. Anzeiger, xii, 

 p. 140 (1889). 



2. Selenka, E. .. .. Report Voyage H.M.S. " Challenger," 



Zoology, vol. xiii, Gephyrea (1885). 



3. Ehlers, E. . . .. .. " Ueber die Gattung Priapulus," 



Zeits. für wiss. Zool., xi, p. 205. 



4. Ehlers, E. .. .. .. " Ueber Halicryptus spinulosus," 



Zeits. für wiss. Zool., xi, p. 401, 



5. Koren, J., and Danielssen, D. C. .. Fauna littoralisNorvegiœ, Beigen, i8yy. 



6. Scharff, R. .. . . " On the Skin and Nervous system 



of Priapulus and Halicryptus," 

 Quart, Journ. Micros. Soc, 1885, 



P- 193. 



7. Schauinsland, H. .. .. " Die Excretions- und Geschlectsorgane 



der Priapuliden," Zool. Anzeiger, 

 ix, p. 574 (1886). 



