234 COMPARATIVE ANATOMY CHAP. 



ventral parapodia. They are found not only in the Ci.t]>it<''U/itit\ but 

 also in Polyophthalmus and the Amphictenidce, and among the OUi/oi'lio-f/i, 

 in the Lumbricullda'. There are many reasons for considering the 

 lateral organs to be homologous with the dorsal cirri of the ventral 

 parapodia of other Polychceta, and in the family of the Gliiccrliln- we 

 can follow, almost step by step, the transformation of these cirri 

 into lateral organs. The cirri, being sensory organs, their gradual 

 reduction into papillae causes the tactile cells scattered in their 

 hypodermis to collect together to form the compact sensory epithelium 

 of the lateral organs. 



Strands of lateral ganglion cells appear always to occur in the 

 lateral lines of the Oligochceta, entering the brain anteriorly. They are 

 closely connected with the hypodermis, and in the Lumbriculidce supply 

 the lateral organs with nerves. They are probably also connected 

 with the intestinal nervous system. 



The function of the lateral organs is at present an unsolved 

 problem. 



F. Auditory Organs. 



There is a remarkable absence of auditory organs in the F'ermes. 

 They only occur in the Polychazta, and there also only occasionally in 

 a few families, viz. in the Arenicolidw, in the TcirlcUida 1 (Lunicc), and 

 Serpalidce (Myxicola, Amphiglene, Fulnic'm}. Their occurrence has also 

 been proved in several Terebellid larvae, in the larvae of Eupomatus 

 (Serpulidd), and in a nearly related Chcetopodan larva. They are 

 paired, and lie in Arenicola on the cesophageal commissures in 

 the head segment, and in this case receive their nerves from the 

 brain. In other forms they lie, as it appears, in the first trunk 

 segment, and are supplied with nerves by the infra -oesophageal 

 ganglion. In this point, as well as in their development, they recall 

 the auditory organs of the mollusca. In adult animals they are 

 vesicular (otocysts), the wall being formed by epithelial cells (sensory 

 cells, auditory cells). The vesicles contain' a fluid in which one or more 

 otoliths are suspended. 



G. The Lateral Eyes of Polyophthalmus. 



Eye-like organs are found in strictly segmental order somewhat 

 beneath the insertion of the transverse muscular bands in the lateral 

 line of Polyophthalmus. They occur in P. pirtu* in the 8th to the 

 19th body segments, and are closely connected with the hypo- 

 dermis which is free from pigment, and which with the cuticle 

 covers each eye. Each eye consists of a lens, a pigment cup, and 

 a body which consists of prismatic cells placed in this cup. The 

 pigment cup and cell body together perhaps form a sort of retina (?) 

 It must be expressly noticed that besides the lateral eyes, PuliiupJi- 

 thalmus possesses cephalic eyes as well (3 in number) and lateral organs, 

 and that it is very doubtful whether the lateral eyes are visual organs. 



