cxxx Introduction. 



Tomopteris, the ciliary movement within these organs has been 

 observed to set inwards and to carry spermatozoa with it ; and this 

 must be the case whenever the ova are impregnated within the 

 maternal body. The nervous system consists of a ganglionic prae- 

 oral mass, and a ventral ganglionic chain, which by their com- 

 missural junction form the oesophageal collar. In the Polychaeta 

 accessory ganglia are developed upon their muscular proboscis ; 

 in Oligochaeta upon the pharynx ; and in Discophora in relation 

 with the jaws (in the Gnathobdelleae), and also with the ventral 

 surface of the digestive tract. In some Annulata the cirri are 

 supplied with nerves^ and would appear to be tactile organs. The 

 eyes are usually but not exclusively carried upon the praestomium ; 

 they may be repeated in each segment {^Myxicola and Polyoph- 

 thalmus), or carried upon the branchiae (some Sdhellae and Tere- 

 heUae), or carried upon the caudal extremity [Amphicora). Bila- 

 terally symmetrical otolithic capsules have been found in a few 

 Annulata [Arenicola, Fabricia, Sahella), in the neighbourhood of 

 the oesophageal nerve-collar. In the Polychaeta, which with the 

 exceptions of Protula, Spirorbis, and Exogone pusilla, are dioecious, 

 the sexual glands develope tliemselves round branches of the pseud- 

 haemal vessels, and discharg'e their products, as in Oligocliaeta but 

 not in Discophora, by dehiscence into the perivisceral cavity, 

 whence they are taken up by specially modified segmental organs. 

 In the Oligochaeta and Discophora, which are hermaphrodite, certain 

 accessory generative organs are present, which are not found, except, 

 possibly, as rudiments, in the Polychaeta ; and the Discophora differ 

 from all other Annulata, in that their generative products do not 

 find their way into the perivisceral cavity by dehiscence, but are 

 conveyed along closed ducts to azygos efferent canals, the terminal 

 segment of the male division of which is modified so as to serve 

 as an intromittent organ as in the Platyelminthes. In development 

 a ' primitive streak,' out of which the various organs both of vege- 

 table and of animal life, with the exception of the digestive tract, 

 are evolved, has been observed to make its appearance ; it appears, 

 however, subsequently, and not, as in Arthropoda, anteriorly to the 

 formation of the primitive embryo. The embryos of the Oligo- 

 chaeta and Discophora undergo no metamorphoses after being set 

 free from the eg^. Those of the Polychaeta are, with a few excep- 

 tions {Amphicora), when set free from the Qgg, furnished with 



