THE POLYCH^TA. 211 



have observed c.'ecal 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 oligochsetous genera, Euaxes and Lumbriculus. 

 The principal trunks give otf a great number of branches, 

 which ramify very minutely in some Annelids [Euyiice) 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 Folychmta 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 JPolychceta. In some 

 cases their walls are thick and glandular, and they probably 

 have a renal function. In addition, they 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 diff'erences presented by the nervous systems of the 

 Polychosta result from the varying len<rth of the transverse 

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

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

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

 the two series of ganglia are fused into a sina^le cord enlarged 

 at intervals. Every transitional condition between these is 

 observable in Terehella^ Aonia, Glycera^ Phyllodoce^ and 

 Aphrodite. In most Polychrpta a very extensive series of 

 visceral nerves supplies the alimentary canal. 



The recognizable organs of sense in the ArineUda are eyes 

 and auditory vesicles. The former are usually very simple, 

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

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

 invariably, with transparent spheroids or cones. Alciope and 

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



