TYPE ANNELIDA. 209 



sometimes imbedded in the dorsal surface of the braiu. For 

 the most part they consist of a cup of pigment-cells, in which 

 numerous sensory cells are present a lens being in some 

 instances developed above each eye. Occasionally, however, 

 as in the pelagic genus Alciope, the eyes reach a high grade 

 of development. In some forms they are not confined to the 

 region of the head, as for instance in the genus Polyophthalmus 



so named from the fact that pairs of eyes are found on the 

 sides of a number of the trunk metameres ; in the majority 

 of tubicolous Annelids eyes are found in considerable num- 

 bers upon the branchial lobes of the head segment, the genus 



Vermilia possessing somewhere in the neighborhood of 11,000 

 separate ocelli in this region. These eyes are simply differ- 

 entiations of the ectoderm, and in many cases are still situated 

 in the hypodermis ; they consist of a number of cells which 

 are prolonged at their inner ends into a nerve filament, while 

 peripherally their protoplasm is converted into a refractive 

 substance, each of these cells being separated from its neigh- 

 bors by pigment deposited in its peripheral layers, as well as 

 by a number of smaller pigment-cells. On account of this 

 pigment-sheath it is presumable that each of these optic 

 elements or ommatidia functions more or less independently 

 of the rest, and the eyes are to be considered as compound, 

 composed of a number of independent parts each of which is 

 physiologically an eye. 



Auditory organs or otocysts also occur in certain forms, 

 but cannot be considered as typical of the Polychseta. In 

 Arenicola they consist of two sacs lying in close proximity to 

 the circumoesophageal commissures and connected with the 

 exterior by a narrow canal, indicative of their origin as invag- 

 inations of the hypodermis. The walls of the sack are formed 

 by columnar cells terminating below in a plexus of nerve- 

 fibrils and covered on the surface turned towards the cavity 

 of the otocyst with a firm homogeneous cuticle, and not pos- 

 sessing any terminal hairs. In the cavity a varying number 

 of spherical particles of carbonate of lime, the otoliths, are 

 found. In some forms a number of such otocysts are present, 

 as in Aricia, where four or five pairs have been found in adult 



