THE POLYCELETA. 211 



have observed crecal branches depending into the perivisceral 

 cavity in which the contained fluid underwent merely an alter- 

 nate flux and reflux. Ramified caeca of a similar kind appear 

 to exist in the oligochaetous genera, Euaxes and X/umbriculus. 

 The principal trunks give off a great number of branches, 

 which ramify very minutely in some Annelids (Eunice) and 

 may give rise to retia mirabilia (Nereis) ; but in many (e. g., 

 Protula) there are hardly any branches and no minute capil- 

 lary ramifications. 



In many Polychceta no segmental organs have yet been 

 discovered, and in others they appear to be represented by 

 mere openings in the parietes of the body. I have observed 

 short ciliated canals opening externally upon the ventral sur- 

 face at the bases of the parapodia in Phyllodoce viridis, and 

 there are indications of the existence of similar organs in 

 Syllis vittata. True segmental organs have, however, been 

 found by Ehlers and Claparede in many Polychceta. In some 

 cases their walls are thick and glandular, and they probably 

 have a renal function. In addition, the}*- frequently play the 

 part of oviducts and spermiducts. Whether the ciliated canal 

 extending along the ventral surface of the intestine, which I 

 have described in Protula, is a structure of the same order or 

 not, I am not prepared to say. 



The nervous system of the Polychceta usually consists of 

 a chain of ganglia — one pair for each somite — connected 

 together by longitudinal and transverse commissural bands, 

 which diverge between the cerebral ganglia and the succeed- 

 ing pair, to allow of the passage of the oesophagus. The most 

 important differences presented by the nervous systems of the 

 Polychceta result from the varying length of the transverse 

 commissures. In Vermilia, Serpula, Sabella, these commis- 

 sures are very long, so that two distinct and distant series of 

 ganglia appear to run through the body, while, in Nepthys, 

 the two series of ganglia are fused into a single cord enlarged 

 at intervals. Every transitional condition between these is 

 observable in Terebella, Aonia, Glycera, Phyllodoce, and 

 Aphrodite. In most Polychceta a very extensive series of 

 visceral nerves supplies the alimentary canal. 



The recognizable organs of sense in the Annelida are eyes 

 and auditory vesicles. The former are usually very simple, 

 consisting of an expansion of the extremitv of the optic nerve, 

 imbedded in pigment, and provided occasionally, but not in- 

 in variably, with transpnrent spheroids or cones. Alcinpe and 

 Torrea have very well-developed and large eyes. The eyes 



